UKH

Im curious,
Is it due to an increase in cohesive forces between the rock particles, resulting in a greater proportion of grains staying put when you slap that sloper?
Sweating less must help.
Greater adhesion, but why?
Could be miles out, can someone enlighten my poor soul?

In reply to The Flying Giraffe:

Sweating less is the obvious one.

There may be more.

In reply to The Flying Giraffe:

Something to do with the rubber on yer boots.. Been done on here before, going into the specifics...

But I cant remember them..

In reply to Daniel S:Its not only the rubbers, you must notice a difference when you hang slopey holds in the cold?

In reply to The Flying Giraffe:

True.. I put this down to less greasiness, and less sweat tho....

In reply to Daniel S:

I believe its due to the fact that particles at the atomic level (in the rubber, rock and probably your skin) are less 'active' in the colder conditions. But as has been said I'm sure this topic has come up before...

In reply to D Berry:

so would I be able to climb b15 at absolute zero ?

In reply to absolutepunter:

or would that require an additional reduction in gravity?

In reply to absolutepunter: I would suspect all the insulation you'd need might ofset the frictional gains somewhat...(assuming you still had some activity at the atomic level at said temp)

In reply to D Berry:

hmm... a minor flaw in my plan...

In reply to D Berry:
I wonder then, why do car tyres stick better when they are warm?

In reply to D Berry: Not that Im a nuclear physicist or anything, but Im a little doubtful that with the temperature changes we're talking about you'd see much difference due to these factors....

In reply to Anonymous: Different rubbers work best at different temperatures. Hotter isn't always better.

In reply to The Flying Giraffe: there is more moisture in the air when it is hot and less in the air when its cold so maybe that has sumthin to do with

In reply to The Flying Giraffe:

it's fewer migrating southern spivs leaving a residue from their slick backed forward facing haircuts.

In reply to The Flying Giraffe: I thought it was something to do with the rock 'sweating' when it is hot due to evaporation. It is supposedly more obvious in porous rocks that hold water. At the crap level I climb, it really doesn't make much difference. It is an unfamiliar phenomenon in Scotland, where there is more moisture in the air than in the lochs! That's why Scottish Gully climbing is so much fun.

In reply to The Flying Giraffe: Car tyres do work better when they are warm - because they are designed too. Climbing route rubber is designed to work optimally at around 4 degrees C (allegedly - presumably because this is about the best temperature for your hands too - not so cold your hands freeze but cold enough not to sweat). At lower temperatures the rubber is too hard and doesn't conform to the rock as well. At higher temperatures the rubber is too soft and conforms too easily acts in a more liquid like manner. This is why temperature is very important on grit, because the crystals are large and the friction therefore depends greatly on the rubber's ability to conform to the rock.

In reply to Glen:

So you can tailor your route to the temperateure, if youre going on a slabby route one week and a non slabby route the other, its best to go do the slabby route on the hotter week as your softer rubbered shoes can smear better?

Yay or nay?

You could do E50 in absolute 0, my tongue sticks to an icepop at about -10, you could stick anything to it if it was -273C. Spose you couldnt take it off though...

It'd Be interesting though, Ill see you all at the Pluto climbing campionships in 4012.

Mat

In reply to matnoo:
but maybe it conforms too easily amd you end up sliding down the route.

In reply to Punter Hitch:


I take it then this would be more of a hazzard in Italy.

I expect that softer rubber would have an optimal temperature that was lower than that of harder rubber, but the exact chemical make-up of the rubber is probably the most important factor.


Sorry, but the reason your tongue sticks to your icepop is that the water on your tongue freezes 'gluing' your tongue to the icepop. Your rubber would be crap at very low temperatures

In reply to Glen:

Just do a spot of DWS, then go for the e50

In reply to The Flying Giraffe:

I thought that as you approached absolute zero you got "superfuidivity" or something which gives no friction at all.

So in addition to the problem of eveything bevcoing very bittle (including you fingers) as you approach -273 degrees centigrade friction would decrese to zero.

I cant help thinking that this is a rather academic argument, although none the worse for that.

In reply to Gary in Germany: IF climbing shoes are designed to work optimally around 4 degess C then whoever makes them is stupid.

How often is it near 4 C in this country. WHy didn't they make it to work best around 15 degrees or 18 ??

And why make it so the shoes stick best when your hands AREN'T sweaty. Surely you need more help with climbing when your hands are sweaty so you can climb easier.

In reply to Glen:

Maybe this subject has been discussed before on RT, but what's being suggested here is clearly wrong i.e that rock boots perform better in the cold. Two reasons:

1. Rock boots are not as far as I know 'designed' to work best at 4 degrees C. It is well known that the first sticky boots (Fires by Boreal) were made of exactly the same synthetic rubber that is used in aircraft fighter tyres. That was the big break through, a superb piece of lateral thinking by the guy designing rock boots for Boreal. I suspect that all modern rock boots are variations on the same totally synthetic aircraft tyre rubber, which is obviously designed to work well at very high temperatures. (Well, all temperatures, if you think about it.) So, nothing to do with the rubber.

2. My experience and the opinion of all experienced climbers is that rock generally (and gritstone in particular) has far better frictional qualities when it is quite cold than when it is warm. The climber's intuitive experience is that this is a property of the rock and not of the boot rubber.

Clearly we need an expert scientific opinion on this.

In reply to Gordon Stainforth: Maybe the tyres/boots should be warm and the road/rock cold so that the rubber forms to the surface then cools and stiffens in that shape ?

In reply to David Riley:


Maybe we should go back to flashing a blowtorch over the soles of his boots prior to setting out on "faith in friction" routes then

In reply to Punter Hitch: Electrically heated thermal setting soles. Another patent.

Damn ! I published it.

In reply to Punter Hitch:

less sweat = less shite/chalk/grease on the holds..

Formula 1 rubbers are alway better in cool conditions as are our boots as the rock itself if cold.

If the rock sweats I'm Gar, ga ga ga ga ga ga... Garah Gates.

The grease is due to the lichen & other plantage in the summer giving off spores/seeds which give the rock the conditions to take on vegetation as its warm..

in the winter the rock gives up that property & therefore has a cooler & cleaner base for the rubber, which if you think about it, hot will stick to cold....

.....then again that all might be shite as usual from my gob.......but sure I posted this before.....

si

Aircraft tyres are generally rather cold at landing (when then need to grip). Also when I said 'designed' I didn't necessarily mean that the rubber was specially mixed for these properties - it may just be chosen for these properties.


I don't see how the properties of the rock can change significatly with the temperature changes in question.


I'm not an expert on rubber, but I am a Physicist, so I know a bit about friction

In reply to Gordon Stainforth:


I don't think this is clearly wrong at all. It's definitely true that rubber friction has quite a sharp peak at a certain temperature which you can change by changing the formulation. It seems fairly plausible that the aircraft tyre rubber that Boreal introduced is designed to run at lower temperatures than car tyres because an aircraft tyre needs to work well as soon as it touches the ground, at which point it will still be very cold.

Two complicating factors, though, are that for car tyres you need to trade off how much the rubber wears against how high its friction is. You can probably tolerate more wear in a rock boot. And the temperature at which the friction is highest depends on how rough the rock is, so if a boot worked best at 4 degrees on gritstone the optimum temperature would be rather higher on limestone.

And the temperature at which the friction is highest depends on how rough the rock is, so if a boot worked best at 4 degrees on gritstone the optimum temperature would be rather higher on limestone.

Exactly. The reason the rubber works best on grit at quite low temperature is that grit has large crystals, which deform the rubber easily when weighted. Therefore a slightly harder and better damped rubber (ie. colder) is better.

In reply to The Flying Giraffe: I'm pretty sure it was Eddie Jordan (though I might be wrong about who said it, but not the position) who said " We would love to be able to use the rubber from climbing shoes on our tyres". Clearly they're not already doing so or it would be a very strange thing to say.

F1 tyres are very soft relative to a road tyre, 'cos you and I can't afford 3 sets of tyres every time we go to the shops and they need adhesion more than durability. They are much harder than the rubber in a rockshoe though, 'cos we don't go over 200mph and through chicanes, so we don't stress our rubber in the same way. They have to balance between durability and performance, so that's why they have a harder rubber and heat it up before thay race.

The fighter tyres are different again because they need to not blow out under extreme forces when cold, but they don't actually do a lot of the braking in the way a car tyre does; landing lines on carriers, parachute braking etc.

3 different rubbers, they don't really stand comparison 'cos they have different functions and properties.

In reply to The Flying Giraffe:

it's also to do with the way your skin behaves at low temperatures. the fats that make up th emembranes in your skin become less fluid at lower temperatures allowing for lesss slippage, damge to your skin etc. that causes you grip to slip.
ta
GTE

In reply to G Tiger Esq.:

Although this is true we are talking primarily about the friction properties of the rock in relation to the feet.

So far the most plausible theory seems to be that the (synthetic, aircraft tyre) rubber performs better when it is quite cool (around 4 C), particularly when the rock is colder than the rubber.

No convincing theory has so far been put forward as to why the rock itself may become more frictional when it is colder - something I was arguing last night, which may well be a complete fallacy.

We still need an expert voice on this!

In reply to Simon:
" the rock sweats I'm Gar, ga ga ga ga ga ga... Garah Gates. "

well I've often seen sweaty rock. Some of it seems to be a reaction you can get on sea cliffs whereby condensation is fromed on the rock. This often happens on very hot days, and is often seen at swanage on the faces and at Sennen deep in the cracks even when it's been hot and dry for some time.

I've also felt gritstone feel sweaty on hot days and it's not just my hands, is it possible that porus rocks such as gritstone can sweat ? I see no reason why this coud not be something to do with evaporation of water inside the rock caused by the rock heating up, adn then the moisture transfering it's self more equally throughout the rock, including on to the surface. I know that one weekend this summer i was burning my leg on a slab climb from the contact with the grit soi it can get very hot. I have felt grit feeling sweaty for no obvious reason.

Another point I've noticed is the shoe rubber can soften so much in the heat that it can start loosing it's predicatabuility.

In reply to The Flying Giraffe:

IMO

1. The friction for smearing is better in warm conditions ( like racing car tires)

2. However, as the rubber is softer when warm, it makes edging less positive as the rubber will deform and 'roll' off the small edges, i.e. Edging is more positive in the cold ( rubber is stiffer)

3. For a particular rubber formulation there must be some optimium temperature where it is sticky enough to smear & stiff enough to edge.

My2P

In reply to Gordon Stainforth:
You're right, I don't think there's any reason to imagine that rock gets more frictional when it's colder.

Rubber is quite different, though; here's what I wrote when this subject came up a few months ago:

Where rubber friction comes from is the fact that if you move rubber across a rough surface, you get a cyclic compression and decompression of the rubber - imagine one spike; as the rubber creeps over it it is alternatively compressed and then expanded as the spike goes past. High friction rubber is formulated so that energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction.

The amount of energy lost in this depends strongly on temperature, the sliding velocity, and on the character of the roughness of the surface, and this rate of energy loss has a fairly sharp peak. So for a given sliding velocity and roughness, there will be a pretty well defined temperature at which the energy loss is a maximum. (If the temperature is too low, the rubber is hard and entirely elastic, so no energy is lost; if it is too high then the rubber is more liquid-like and can easily flow out of the way) . Being more technical, the response of rubber to a cyclic deformation is described by frequency dependent complex modulus. The energy lost per cycle of deformation is referred to as tan delta, and it (like both real and imaginary parts of the modulus) is a function of the product of frequency and the temperature dependent "shift factor". The relevant frequency here is the sliding velocity divided by the length scale of the roughness (real surfaces would have a distribution of such lengthscales so you'd have to integrate over all these contributions). Tan delta for most rubbers has a strong peak which for normal testing frequencies is about 50 degrees above the glass transition of the rubber.

In principle you ought to be able to design a rubber that has maximum friction not just for a given temperature but also for a given rock type, but I very much doubt that rock boot manufacturers have done this. The job of someone rationally designing a rubber formulation is to make sure the tan delta peak occurs at the temperature you want for a given type of surface roughness. You would do this by changing the composition of a copolymer, changing the cross-link density and sticking some additives in.

If you feel the need for more detail, see "Theory of rubber friction and contact mechanics", B.J. Persson, Journal of Chemical Physics 115 p 3840 (2001).

In reply to Gordon Stainforth:
read the question you monkey
don't slag off my answer just cos you don't want to talk about it. the question was not just about boots, it turned into a discussion on frictional properties of rubber, but with a phrase like "slapping that sloper" in the question i'd say it pertains to hands as well as feet.
so don't be such a cock
cheers
GTE

In reply to G Tiger Esq.:

you look like an idiot for being so unnecissaily rude to someone. completely uncalled for.

In reply to G Tiger Esq.: If its true that different rubber works best at different temps, andobviously it is cos of F1 tyres then we should have different boots for summer and winter, or even for different places. Surely the same rubber cant be the optimum for both limestone and grit?
Or even for different countries, "Spanish boots of Spanish rubber" I feel a song coming on

In reply to Al Evans:
Yes, I see a day when we have summer grit boots, winter limestone boots, spanish holiday boots ... even if the science won't work I'm rather afraid it's a compelling marketing idea.

In reply to "frequency dependent complex modulus":

All this talk of racing cars and aeroplanes is just completely off the mark. This is about slopers, friction and science. Theres only one man who can help us....

Take a moment to read the following article:

http://fatsloperaction.com/eric.html

As you can see from the text, the 'frictional hang coefficient' is strongly dependent on air temperature.

Surely the temperature effect on the friction of your hands is far more important than that of your feet.

In reply to Richard J:
", and it's this energy loss that causes friction. "

Is that strictly true ? I would have thought energy loss through friction could happen a number of differing ways. For starters the mechanics for moving and non moving friction are different (the shoe rubber will not always be creeping, or the creep will be so slow as to be neglibable).

Secondly I would have thought rubber already pre compressed will still cause energy loss due to friction. Imgaine a smear on a perfectly flat sloping hold.

I take your point though that most of the energy might well be lost this way.

In reply to Woker:
That's a perceptive point. The consensus seems to be amongst people who study rubber friction that there is no such thing as true static friction for rubber, and you always get some creep. The argument is that creep is never so slow as to be neglible, because what matters is not the absolute speed with which the rubber is creeping, but the rate of deformation of the rubber, which you get by dividing the speed by the distance between the microscopic protrusions of the rock. And there will always be very very small scale roughness that will translate an impercetibly slow rate of creep into a significant deformation rate.

I think what I'm saying is the conventional wisdom in the field, but that isn't to say that it's the complete picture.

In reply to Richard J:
Interesting, I suppose you'll argue that a perectly smooth surface would have zero friction which ofcourse is impossible, there agasin it's impossible to have a perfectly smooth surface... Hmmm interesting.

In reply to Woker: Static Friction is always higher than sliding friction (the reason F1 cars (back to them again!) try to avoid rally style antics). At a fundamental level friction is due to the various electro-static forces in the structure of, and between, the atoms in both surfaces. The ability of rubber to deform increases it's contact area on a microscopic scale. Thus more bonds between atoms in the two surface (and to a minor extent between the surfaces) must be broken in order for the sufaces to slide. Thus the friction (between rubber and rock) is higher than if the rubber were a hard surface.
Friction will cause energy to be released by virture of breaking atomic bonds. This energy is principally released as heat (eg. rubbing sticks to light fire).
Energy loss does not cause friction but is a result of the process.

In reply to Richard J:

OK, there seems to be some consensus now among the 'experts' here that warm rock combined with warm rubber results in less friction, but I don't think we have fully explored the question of the rock itself. Someone suggested the perfectly plausible theory that a porous rock like gritstone itself 'sweats' a bit in hot weather, and that this dampness reduces the friction, both for hand and foot. No one took him up on that.

In reply to Glen: My goodness. At last someone has got the correct answer! I have never seen so much hot air and bull shit on a thread before now!

In reply to Gordon Stainforth:

God one day decided that he liked the pictures of bouldering in the winter better than the summer, so is trying to make us all look shit in the summer....

its true he told me..

S

In reply to The Flying Giraffe: The creep of rubber metioned is a real effect. This is why the friction of rubber on a hard surface does not increase linearly with normal (technical usage ie. at right angles to surface) force, as it does with hard surfaces. Creep causes reduced (compared to linear expectaion) friction at high force/pressure. This is why F1 tyres (again!) are so wide - increase surface area gives reduced pressure between the surfaces.

In reply to Glen:
"ergy loss does not cause friction but is a result of the process. "
yes I know that and stated as such in my post I beleive.

Interesting about the electro static stuff, I was thinking it'd be more to do with small particles chipping and breaking and objects having to move up and down over bumps, that would cause friction and the energy loss associated with it.

In reply to Glen:


This can't be right. Imagine moving two perfectly smooth surfaces past each other. To make the surfaces move, you have to break a pile of bonds, which cost you energy. But when the surfaces stop moving you make an exactly equivalent set of bonds which give you back exactly the same energy that you had to put in. To get friction, you've got to have a mechanism for absorbing energy irreversibly.

Basically the same thing. The small particles/bumps/dips are held together by electrostatic forces (chemical bonds)

In reply to Richard J:

Yes, but you are constantly putting energy into the system by providing a force to move the two surfaces past each other. Work is done to move the surfaces and energy is transfered from the pushing device to the sufaces (which get hotter).

In reply to Glen:
You only need to put a force in if there is friction there - your argument is circular!

In reply to Richard J: How can there be no friction? How can we have 'perfectly smooth surfaces' when we are talking about atomic scale forces?

In reply to Simon:

While you're at it, then, ask Him how he makes the rock have more friction when it's cool. He could be a useful contributor to this discussion.

In reply to Glen:
The key point is that the atomic scale forces are conservative... you can describe them accurately by a potential. That means (and I'm taking you at your word when you say you're a physicist when I put it this way) that the Lagrangian of the system has no explicit time-dependence, energy is conserved and the equations of motion are equally valid whether you run time forwards or backwards. But friction is a dissipative force, which depends on the velocity (not just the position and the acceleration) and in a system with friction in solutions to the equations of motion are very definitely not the same when you run time backwards and forwards. So you can't derive friction from a completely conservative set of equations of motion.

This is the same general argument as to why you can't derive the second law of thermodynamics from classical mechanics.

In reply to Glen:

Sorry, that's not strictly true.

contact area and static friction are not proportional.

1kg weight with a contact area of 1cm² will have the same overall static friction as 1kg with a contact area of 1m².

The softness in the rubber does allow the rubber to morph to the rock better, but this simply means that there is a more mechanically sound interlocking.

This even goes to the F1 car. The rubber in contact with the road is not sliding, so the minor deformations holds true here also.

The reason that kinetic friction is less is than once the deformed rubber slides out of the pits, you get peak to peak interaction, which is more like the atomic bonds business you were talking about. The surface morphology acts as traction and not friction.

In reply to Al Urker: I don't think I said that contact area and static friction were proportional. A least I didn't mean too - that's obviously not true. I was trying to describing the same traction affect as you.

In reply to Al Urker:

On this one I agree with Glen. What you say is true for friction of other materials like metals, but rubber is different. None of what we think of as the laws of friction (friction force proportional to normal force, independent of contact area, independent of velocity ) are even approximately true of rubber. This is because the mechanisms of rubber friction are quite different from the friction of all other materials.

In reply to Richard J:

That's exactly the point I was trying to put across. How people view friction is all well and good when you describe a non-distorting surface, such as a metal block sliding across a concrete floor, but deformation changes things slightly

In reply to Richard J:


You are correct when you say this. However, I wasn't trying to mathematically derive friction (for rubber or hard sufaces). I was just trying to provide an basic argument to explain why frictional coefficients invoving rubber are temperature dependent (which you were also stating - I think).

We are all arguing the same thing!

In reply to Al Urker:
All friction involves deformation, but whereas in two blocks of metal sliding across each other the deformation is very highly localised, with plastic yield at the points of contact, in rubber, because it's so much softer, the deformation is spread quite far into the bulk of the material.

In reply to Richard J:

That's not quite the deformation I was meaning. yours is a result of the friction, mine is what causes the 'hook up' of the granularity of the rock. Friction is still present in all cases.

Do you know, one picture would sort this sort of misunderstanding out.

In reply to Richard J:

woops! Just noticed that I was somewhat unclear here. The energy is not released by breaking bonds - quite the oposite of course since the electrons occupy a lower energy state in a 'bond'. I failed to fully explain that the energy release comes from the subsequent new bonds. Energy is put in to break (and/or stretch) bonds via a force (to 'overcome' friction and move the surfaces against each other) and then is released (as heat) when new bonds are formed (and/or bonds relax). I think this is where our disagreement originated

In reply to Glen:

Look, there have been 67 answers so far, and we don't seem to be any nearer answering the original poster's question, which is: why does the rock itself seem to have greater frictional properties at lower temperatures? Not: why does sticky rubber adhere to rock better at lower temperatures (which question has provoked an incredible babble of scientific jargon).

Woker made the suggestion that a porous rock like grit actually sweats a bit in hot weather, but no one followed this up.

In reply to Gordon Stainforth: What part of "why does colder weather result in greater friction" specifies rock and not rubber?

In reply to Gordon Stainforth:
"Woker made the suggestion that a porous rock like grit actually sweats a bit in hot weather, but no one followed this up. "

Well i suggested it could be that way that's for sure. Is this possible any one know ?

I should imagine it's just condensation forming on the rock somehow, but maybe it sweats too.

In reply to Gordon Stainforth:

I think the overall reason is in the hands and not in the rubber.

and most of it has probably to do with sweating causing the oil in hands to come to the surface, when applied to rock forms a very poor bearing surface, but a bearing nonetheless. Cold weather = less sweat, ==> less oil brought to surface, less slipperyness.

In reply to The Flying Giraffe:

So then as a result of all the above how long before a manufacturer brings out a range of shoes specific for rock and temparature.

They could convince us that we need loads of pairs of different shoes rather than one pair we get resoled every now and then.

I am such a gear freak and so desparate to push my grade that i would proably buy them all!!!

In reply to Al Urker:
I think there is some truth in this, But the rock it's self can appear to be sweaty when hot too. I've seen this many times climbing on sea cliffs.

In reply to Gary in Germany:

Nah, shoes are expensive.

If that really was the case, then interchangeable sections of sole would be the way forward.

But the winning shoe would be that with the rubber with the widest range of temp.

I think a shoe specifically for DWS will come out soon - drainage mesh etc.

In reply to Woker:

I've done a lot of damp limestone, which doesn't seem to serioulsy affect performance.

Is this 'sweating' of the rock a greasy film of sorts then?

In reply to Anonymous:

well, he enlarged on the question in his message, asking about the 'cohesive forces between the rock particles, resulting in a greater proportion of grains staying put when you slap that sloper'. He was clearly talking mainly about the frictional properties of the rock itself.

In reply to Woker:

I'm going to suggest another possible reason for the rock surface being less 'sweaty': that cooler air holds less water/ is generally less moist.

In reply to Gordon Stainforth:
the fact that cooler air holds less moisture is not completly true. What is true is that warm air holding the same amount of moisture as cold air in terms of water weight will have a LOWER humidity reading. Humidity is what you should be talking about not moisuture per se. So given some warm air if you cool that down the humidity will actually rise.

However due to sunshine and warmth evaporating water I'm assuming that mean himidity readings between summer and winter will show that the humidity is higher in summer. However i don't really see how this is likely to significantly effect the dampness of the rock unless it actually condenses on it. Higher humidity however will make you sweat more.

Condensation happens when warm air meets cold air so basically the rock would have to be either be much warmer or cooler than the surrounding air, and I'm not sure on the actual physics going on here to whether it musty be one or the other so will keep quiet. However I suspect rock get's condensation on it when it's a fair bit cooler than the air around it but I could be wrong.

In reply to Woker:

Speaking as a physicist, in reply to your post, I can wholeheartedly say





























AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARgh!

In reply to Gordon Stainforth:
Gordon, it was you that stated that the opinion that rock boots worked better at low temperatures was "clearly wrong" and then you said you wanted an expert opinion on the subject. Well, you got some opinions, and I put enough scientific detail into mine that someone informed would be able to judge whether it is expert or not. Just because it wasn't in accord with your preconceptions and/or you weren't able to follow it in detail doesn't mean you can dismiss it as a babble of scientific jargon.

I'd have thought it was fairly obvious that it's a senseless question to ask whether "rock itself" has greater frictional properties at low temperature. Friction is made up of the interaction of the thing that is sliding and the thing that is being slid on. The question "what is the sound of one hand clapping" is good zen but bad acoustics!

In reply to Richard J:

Overall I think it's been quite an interesting discussion. But I also think it's got somewhat bogged down in scientific minutiae. What I would really like to see is a scientist explain this question of friction of rock on rubber in ONE very clear, well-written sentence.

Also, the important other part of the question re. the frictional properties of rock itself at different temperatures doesn't seem to me to have been addressed at all, except by Woker.

I don't think I had too many preconceptions on the subject, and you will probably have noticed how I changed my position once I'd read various arguments.

Hang on! I accept that my use of language here was sloppy. Of course friction is about the interaction of two surfaces. What I think a lot of climbers would be interested to know is if anything about the rock surface itself changes in cold weather that makes it more frictional in relation to an unsweaty hand, for example.

In reply to Gordon Stainforth:

Here's an edited version of the two sentences I started with:
"Where rubber friction comes from is the fact that if you move rubber across a rough surface, you get a cyclic compression and decompression of the rubber - imagine one spike; as the rubber creeps over it it is alternatively compressed and then expanded as the spike goes past. Energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction."

What would you like me to explain further in that?

In reply to Gordon Stainforth:


Well, four RT contributors were having an entertaining time trying to sort out the fundamentals of what's a difficult and subtle scientific problem. I'm sorry you couldn't participate fully. But I notice that you're quite happy to take quite an elitist line on the thread about music, so I don't see why you should complain when other people take an argument in an area you are not so familiar with to a deeper level than you are comfortable with.

In reply to Richard J:
Not bad for two sentences. Yes, I see that. Can we now incorporate what happens when the rubber gets hot? Presumably the rubber is compressed and expanded rather more easily with less resultant friction. ?

In reply to Richard J:

Good that you were thrashing the subject out, but it was not something most people would have found very easy to follow. No comparison with the music example at all. Nothing elitist about classical music. All western music uses the same diatonic scale etc. Very, very accessible, just a bit different in style - but not much. Mostly a matter of quality. You don't need specialist language or knowledge to talk about and appreciate so-called 'classical' music. Which was just the very best music written about 2-300 years ago.

Definite sense though that music may have peaked out then. Just as painting probably peaked out well before 1900, probably well before 1800! And the cinema, I dread to say/think it, may have peaked out in the late 60s. Jazz in the 50s. (Writing is interesting, because it never really dies. Goes through lots of rebirths, and remains always accessible.)

etc

In reply to Gordon Stainforth:

This bit needs a (thought?) experiment. Find one of those hard rubber "superballs" from a toyshop. Bounce it onto a hard surface. The difference in height between where it bounces to and where you dropped it from measures the loss of energy for a cycle of compression and decompression. For that sort of hard rubber, this energy loss is fairly small, so it's frictional properties would be poor.

Now find some liquid nitrogen (from a friendly vet, if you haven't got a nearby school or college science dept). Dunk the superball in a beaker of the stuff until it stops bubbling. Pick it out (since it's now at around -200 C this is a good time to put your Dachsteins on). The ball will now be quite hard and glassy, much like a marble. Bounce it on the hard surface again. Since it's now quite brittle, there's a chance it will shatter, but if it does bounce it will, like a marble, bounce to a significant fraction of the height you've dropped it from. Once again, it doesn't lose very much energy in each cycle of compression and decompression, and as you would expect it's not going to be much good for a shoe sole.

Now let the ball warm up, dropping it every now and again to check out its bounciness. You'll find that as it warms up it first gets less and less bouncy, and then it starts to get more bouncy again as it gets to room temperature. At its least bouncy, you'd find the ball felt rather leathery. This is the point at which the energy loss per cycle is maximised. At this temperature its frictional properties would be pretty good.

Every rubber has a point at which this energy loss is maximised, but the temperature at which it happens varies from rubber to rubber.

In reply to Richard J:

I think the reason this is so difficult is typical of much of materials science. A lot of materials are very complex in their behaviour and counter intuitive in many examples. Its what attracted me to the subject in the first place as a kid, although I'm getting rusty now.

Someone did some lab experiments on slate a few years ago and found water didnt affect friction that much. They forgot about lichen on the rock and mud on the rubber when they extrapolated the results to climbing!

In reply to Al Urker:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARgh!

In reply to Richard J:

Energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction."

I'm still a bit confused about this statement (sorry Gordon!). Is this not hysterisis loss you are describing? Although this is closely related to friction (internal frictional heating), I don't see how this affects the friction between the two surfaces.

Woops. Didn't mean to be anonymous.

In reply to Anonymous:
so what your saying is all this sci-babble could of infact been utter bollockss in the first place ?

In reply to Anonymous: Oh. I suppose hysterisis loss in the surrounding the areas of the rubber surface which are distorting around the rock crystals would increase sliding (and 'static' ie. creeping) friction.

In reply to Woker: No I don't think so. We have just been misunderstanding each other I think. RichardJ should have said 'compression/decompression cycle accounts for the different frictional properties of rubber, compared to hard surfaces'. I think just stating "causes friction" was misleading (to me anyway)

In reply to Glen:
Yes, this is what I am saying, only in a stronger form. Hysteresis loss in the volume of rubber which is being distorted around rock crystals not only increases friction, it is in fact the dominant contribution to friction for rubber.

In reply to Richard J: Good. I'm glad we got that sorted out!

In reply to Woker:

All sci-babble has to be potentially bollocks. If its not disprovable (potentially wrong) its not a scientific theory.

In reply to Glen:
Ok then now we have that sorted whats does anyone think about damp/sweating rock and how this can happen in the absense of rain or drainage ?

In reply to Offwidth:
And as I stressed to woker above, the complicated answer I gave to this apparently simple question doesn't purport to be the absolute truth, it's simply the current scientific consensus for what the most important factors are in a complex situation. It is undoubtedly still quite incomplete as an explanation, and we can all think of things not properly taken into account.

But it does help us understand the central issue, which is why the friction of rubber as a function of temperature has a strong peak. And that's not a theory, it's an experimental fact.

In reply to Woker:

Exactly, Woker - when are they going to address this question, asked about 3 days ago now, I think?

In reply to Woker: Well, it does seem posible that a particuarly porous rock might 'sweat' in warm weather. But I don't think grit is very porous (any geologists around?).
Condensation seems a more likely culprit. Although the amount of condensation would depend on the humidity and the relative temperatures of the rock and the air. I'm not sure if this would result in generally less condesation in colder weather.

In reply to Richard J:

Look, we've talked about rubber (and how!) - can we just get on to rock friction per se, in different weather conditions, irrespective of rubber, or the kind of surface touching it.

In reply to Gordon Stainforth: There is no such thing as "rock friction per se, in different weather conditions, irrespective of rubber, or the kind of surface touching it". The different parts of the system cannot be separated in this way.

In reply to Glen:

OK, fair enough. But when the friction between rock boot and rock seems to improve in cooler weather, have we established that this is simply to do with properties of the rubber? Have we ruled out the possibility that anything can change in the properties of the rock itself?

In reply to Gordon Stainforth: I think the consensus was that water in/on the rock probably plays a part, but how big a part I don't know. Changes in the actual rock (ignoring water) seem unlikely.

In reply to Richard J: Why is hysteresis a factor in this? Surely hysterisis is about the 'memory' in plasticity? I dont see how this can affect rubber sticking on holds except in the long term, ie as the rubber deforms to fit the rock it will 'remember' plasticlly a certain amount of the distortion required of it and stay a certain level back from its original performance. Surely this woulkd be a bad thing?

In reply to Al Evans:

True. I have to agree with Al here.

hysteresis is a bulk response to deformation. Friction is a surface or interface property, connected with the ability to move one surface past another.

There's no reason for the hysteresis in the compression cycle to affect friction in the slightest, unless it is due to heating of the rubber, which modifies the surface of the rubber in some way. If the heating of the rubber causes easier deformity of the rubber, then we're dealing with traction, a mechanincal interlocking on a reasonably large scale.

In reply to The Flying Giraffe:

Also, from personal experience, the low temperature has very little to do with the friction of boots, but may have some benefit to the hands.

In reply to Glen & Richard:
To what level have you studied physics/applied maths ?

In reply to Woker:
PhD, followed by about 15 years of research experience.

In reply to Al Urker:
You assert that friction is purely a surface phenomenon. I assert that (only for rubber) it reflects bulk behaviour. But I've got evidence for my assertion. If you plot rubber friction versus temperature you get a peak that exactly mirrors the peak you get in a plot of energy loss per cycle. what's the evidence for your assertion?

In reply to Al Evans:
I agree that hysteresis isn't maybe the most helpful term here (which is why I only used it when Glen introduced it) because of the connotations it has in magnetic material of memory of structural rearrangements. In rubber the energy loss comes not from any well defined structural changes but simply from non-localised viscous losses.

In reply to Gordon Stainforth:
Since even I am beginning to feel a bit jaded about rubber friction, I'll throw in a guess. The amount of water vapour the atmosphere can carry is a strong function of temperature. On cold crisp winter days, I suspect there is a much bigger variation in temperature between day and night than there is in summer. In the night it gets cold, resulting in the morning mists that are such a common feature of the upper Derwent valley. Then, as the day rapidly warms up, the water content of the air is much less than the saturated vapour pressure, no water condenses on the rock which stays very dry. But in the summer, the daytime temperature isn't so different from the night time temperature, so if the rock stays a little cooler than the surrounding air so some water can condense on it.

As I say, only a guess.

In reply to Al Urker: I agree with Richard (especially now that I understand what he is saying!). For a elastic material such as rubber surface and bulk effects cannot be separated. If the heating involved in suface deformation (hysteresis) is not related to friction where does the energy come from?

In reply to Richard J:

Friction is a purely surface phenomenon because subsequent layers are only in contact with the first layer and not the rock in question here.

I'd be incredibly surprised that if you spaced the block away from the rock, the long range forces that suddenly seem to exist for rubber would still maintain a degree of friction.

If your assumption that the energy lost per cycle causes the increase in friction is correct, then it appears that the hysteresis of the rubber is heating the surface slightly, which is causing a change in the morphology or chemistry (or both) of the surface, which is more akin to the surface of the rock in question, giving a greater chance of interatomic bonds occurring, or minute mechanical hookup.

Also, if this is true, then maintaining the hold would allow the rubber to cool, thereby lessening the friction, and rendering the rock 'slippier'. It would point to a limit where moving footholds would be necessary in order to maintain sufficient friction.

Also, this seems to be contrary to the opinion that cooler rock promotes friction. Therefore the rubber in colder climates probably has less effect than the improvement of hand friction.

I think the key was that, according to richard, the rubber would constantly be creeping, resulting in a constant 'hysteresis' (if we can still use this word) loss.

In reply to Richard J:
explain this then what you say is :
"Where rubber friction comes from is the fact that if you move rubber across a rough surface, you get a cyclic compression and decompression of the rubber - imagine one spike; as the rubber creeps over it it is alternatively compressed and then expanded as the spike goes past. High friction rubber is formulated so that energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction. "

you then contradict/correct as instead of saying it's this energy loss that causes friction you then claim that it''s only the dominant factor :
"!Yes, this is what I am saying, only in a stronger form. Hysteresis loss in the volume of rubber which is being distorted around rock crystals not only increases friction, it is in fact the dominant contribution to friction for rubber."

I do not think however that the deformation of rubber to the surface which then causes the shoe to be in Hysteresis is as you claim is the only/dominant factor in the overall frictional force. Remember you didn't say the the rubber hugging the rock caused more sliding friciton you said "you get a cyclic compression/decompression cycle, and it's this energy loss that causes friction. " I think the fact that the rubber is now hugging the shape of the rock is the reason that the sliding friction of the rubber against the rock is greatly increased .

Also the fact that the rubber is now hugging the shape of the rock allows you to get a better grip upon the rock to push off. I add two quotes to help back this up :

Check this
http://www.aps.org/BAPSMAR98/tocW.html#SW22.004
"Rubber exhibit unusual sliding friction. When rubber is slid on a hard, rough substrate, the surface asperities of the substrate exert oscillating forces on the rubber surface leading to energy ``dissipation'' via the internal friction of the rubber. I estimate this contribution to the friction force and compare the results with the experimental data of Grosch. Because of its low elastic modulus, adheasion of the rubber to the substrate is very important. I show that the adheasion force will deform the rubber at the rubber-substrate interface in such a manner that, at low sliding velocities, the rubber completely follow the short-wavelength surface roughness profile. This gives an additional contribution to the friction force which is estimated and compared to experimental data. See B.N.J. Persson, ``Sliding Friction: Physical Principles and Applications'' (Springer, Berlin Heidelberg 1998) for the complete theory. "

and also this
http://www.msgroup.org/TIP084.html
"While 'stick' is essentially the same as friction, 'grip' is not. Grip is a result of the deformation of the tire's rubber that allows it to 'surround' irregularities of the road surface. This surrounding effect provides more surface contact between the rubber and the road, and it adds the ability to 'push against' those irregularities rather than simply rely on the coefficient of friction between the surfaces. (Tread provides yet another way to 'surround' irregularities in the road surface, thus it contributes to traction as well as providing water shedding capability.)"

Is this not what we have just been saying?


I think the second quote is a fairly simpistic view.

In reply to Woker:
Well, I'm glad I've captured your interest on this one. If you go back to my original post you'll note that I cited Persson's paper on the theory of rubber friction. I suggest you go and read it. It is, of course, loads more complicated than the simple picture I've been putting across, because it deals with the reality that surfaces aren't characterised by roughness on a single length scale; the big bits of grit are themselves rough on a smaller length scale, and the rubber probably can't conformally follow the roughness on all scales. Persson deals with this by modelling the surface as a fractal. But it's still building on the basic classical picture I've been trying to explain (which is due to David Tabor's work in the 50's - his classical work "Friction and lubrication of solids" is still in print).

In reply to Richard J:
Look I'm not saying your compltely wrong, you are obviously wrong on this point
"High friction rubber is formulated so that energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction. " "

When you mentioned it's this "energy loss that causes friction", you were talking about all the friciton/grip in the system at this time not just one part of it. I think the quotes and my reasoning shows this to be wrong, but heck you knew you were slightly off on this one or you would not have corrected your self. But anyway I think you will not be able to prove that Hysteresis is the energy loss that causes friction, whicgh is the only force keeping the boot on to the rock. I agree that Hysteresis allows more grip and sliding friction in the system but that is not what you were saying, or if you were you were saying it rather badly. I also agree that friction internally in the rubber of the shoe is casued by Hysteresis. I beleive that it is traction and sliding friciton which mainly keep the boot on the hold given that the sole of the boot has now pretty much conformed to the hold. If there was only Hysteresis as the force keeping the boot on then you would be relying entirely on traction/grip to keep the boot on the hold and I do not believe this to be the case. In any case even if it is only traction keeping it on, in this case you both also failed to mention this as the main reason the boot stays on the hold rather than friction forces playing the main part.

Also instead of just mentioning the names of some very specialised books, I ask you to provide quotes or links to any page which can cast doubt upon the points I and others have made and make these points in a form of english that can be readily understood.

In reply to Richard J:

Sounds like quite an inspired one. Certainly the most coherent attempt so far to answer that part to the question. What do you think of Woker's suggestion about moisture content of the rock perhaps coming into it as well? If so, that would mean there are three variables, all having an effect on temperature in different weather conditions: Saturation/humidity of air, water content of rock, frictional properties of rubber on rock at different temperatures, irrespective of moisture.

I guess we could write a thesis now on the question of rubber on rock, with some of the expert replies today.

In reply to Glen:
I think this point :
"This Hysterisis gives an additional contribution to the friction force"

contradicts richards point that

""High friction rubber is formulated so that energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction. "

as my quote clearly says there are other friction forces in the system. Richard clearly said after my first post that the only friciton in the system was internally in the rubber of the shoe, this I do not belive to be true and I ask him to prove it. If he finds this impossible I ask only that he can prove that it is the dominant fricitonal force in the system as he mentions later.

also I think this point
"While 'stick' is essentially the same as friction, 'grip' is not. Grip is a result of the deformation of the tire's rubber that allows it to 'surround' irregularities of the road surface. This surrounding effect provides more surface contact between the rubber and the road, and it adds the ability to 'push against' those irregularities rather than simply rely on the coefficient of friction between the surfaces"

shows that it is not just friction which keeps us on the rock but also traction/grip, from the fact that the shoes now fits the shapes of the rock, which up to now has not even been mentioned by you lot, as the boot has conformed to the shape of the rock over the entire boot, you get more traction as any single part of the boot is less likely to shear.

In reply to Woker:

back to the original question a few days (it seems longer) ago...

I'd say Alex's reply: Sweating less is the obvious one.

is mostly right. I'm just back from Font. it was bloody hot 30 degrees most days. I was climbing with a German guy who wasn't using chalk, and who didn't seem to suffer from sweaty hands - even in that heat! He sent a problem that required the use of a poor sloper. When I tried it, my sweaty hands, covered in chalk, just couldn't hold the sloper - the chalk and the sweat had produced a slippy 'paste'. In colder weather I guess less sweat and use of chalk would mean better friction...?

In reply to Woker:
To repeat my reply to you yesterday:
"I think what I'm saying is the conventional wisdom in the field, but that isn't to say that it's the complete picture. "
A scientific consensus isn't the same as God's given truth. The evidence for this point of view is the experiments I mentioned above (which are cited in Persson's paper). But I'm sure they are incomplete, and we can't say more until someone gets out and does some more experiments. It's a good topic for a PhD thesis. Did I hear Gordon volunteering?

In reply to Richard J:

No way, baby! But, if anyone had to write on this subject, all the material is here in this one enormous thread, it seems to me.

In reply to Richard J:
well can you provide some quotes from the research to back up the points I just brought up ?

You can't write an authoratative scientific report based on the fact, "go read the papers". When you make a point you must back it up. I'm not asking for cast iron proof just any link or quote which agrees with you on the points I just raised about your statements, from either an informative website of some kind or some book or other. Since you claim to have gleened this knowledge from such research this should be an easy matter for you.

I think we all understood anyway that the rubber on our shoes conformed to the shape of the rock to give better traction. Obviously depending on the shape of the particles on the surface of the rock this could depend on either very little traction and mostly friction or vice versa, or there could be equal amounts of both.

The point I think you made which I think was most wrong is that hestersis was the dominant factor in the friction of the system which you implied was the force keeping you on the rock and I beleive in general this not to be true. Although temperature and the rate of hestersis obviously play a big factor in traction/grip. I believe I could draw you some profiles of a rock surface that would still rely mainly on friction (not traction) to keep the boot on the rock and that the main friciton in the drawings I'd draw would be between the boot and the rock not inside the rubber as this is already stuck together and the boot and the rock are not.

In reply to Woker:
Honestly, I'm not being willfully obscurantist in my referencing - the papers that express the point of view I'm putting across are research papers. If I knew of any source that put it in simpler language I'd tell you, but I don't (and I suspect I've done the same google searches as you anyway). What I've been trying to do is to translate the concepts in the research papers as simply as I can. I'm sorry if I haven't always been successful, it's a difficult job. I don't take your criticisms personally, because it's not my own research. What you're saying isn't a priori ridiculous, it just isn't backed up by what experiments and theory are out there. And let me repeat yet again, the problem is by no means sorted out, and if you did a load more experiments you would probably find out some more subtleties and corrections not accounted for in the simple picture.

Ok then let me put this to you simply.
If the rubber is sliding over a hold and as it's doing so there is a compression/decompression cycle, there must be some sliding friction as the shoe is sliding over the hold. You have suggested in your postings there is no sliding friction, I have followed numerous links on the net and they all suggest that (sliding) fraction exists when rubber moves over a surface.

I challenge you to find one legitimate looking source you can QUOTE the relavant sections of text from, this could either be a web page or book or paper. Merely mentioning the name of a study paper as you well know does not demonstrate you are correct, you are only human and could have made memory mistakes on the matter. I have at least provided sources to suggest that some of what you said is not true. If you can not do this I will be of the opinion you were liekly to have been wrong in some of what you said.

Most people on this site are intelligent indivduals and I can understand complex math and physics so don't worry about putting it too simply if you can not. Just make sure you quote enuf text to get your point across.

It's not that I don't believe you, but after reading your posts and other information on the net, it would appear the sliding friction exists when rubber moves over a surface and in significant amounts. I believe that most of your post was correct however I do not think you will find a quote or link which says rubber can not exert a frictional forces when it is moved over other surfaces.

Also I think that your posts were highly misleading talking about the internal friction in the sole playing such a major part in the system as I believe the friction of the sole on the rock and also the traction that the rubber gives when it conforms to the shape of the rock are also major sources of the shoe being able to grip/hold on to/exert a frictional force upon the rock.

Please do not reply with the names of a paper etc, if you can not back up your reply with the quotes from that paper or backup your claims with links to web pages. The reason I have got in to this subject is that I was interested in what you were saying, so started looking it up on the net. Every link I found seemed to contradict your theory of the internal friction in the sole being the only/dominant factor in the frictional forces, so I am asking you to prove this, it would seem you cannot.

I also am not taking this personally but would like the matter resolved properly and am quite prepared for the eventuality that I could be wrong/confused.

In reply to Woker:
Somehow I suspect you are misunderstanding me. When did I say there wasn't any sliding friction? I just said that the origin of the sliding friction was predominantly due to the bulk deformation of the rubber that was caused by it being forced to move over the protrusions in the rock.

To meet your challenge, these words are directly quoted from the introduction to the Persson paper I cited above.
"The pioneering studies of Grosch2 have shown that rubber
friction in many cases is directly related to the internal
friction of the rubber. Thus experiments with rubber surfaces
sliding on silicon carbide paper and glass surfaces give friction
coefficients with the same temperature dependence as
that of the complex elastic modulus E( v) of the rubber. In
particular, there is a marked change in friction at high speeds
and low temperatures, where the rubber’s response is driven
into the so-called glassy region. In this region, the friction
shows marked stick-slip and falls to a level of m'0.4, which
is more characteristic of plastics. This proves that the friction
force under most normal circumstances is directly related to
the internal friction of the rubber, i.e., it is mainly a bulk
property of the rubber."

In reply to Richard J:
This is a copy of your post relating to the friciton on climbing shoes.

"Where rubber friction comes from is the fact that if you move rubber across a rough surface, you get a cyclic compression and decompression of the rubber - imagine one spike; as the rubber creeps over it it is alternatively compressed and then expanded as the spike goes past. High friction rubber is formulated so that energy is lost in this compression/decompression cycle, and it's this energy loss that causes friction. "


Evidentally friciton is caused in the system also from the rubber moving over the rock as it moves in a cyclic motion. So your now willing to accept that this statement
was wrong then ?

Even the quotes in your last post agrees with this as it states :

"In particular, there is a marked change in friction at high speeds and low temperatures, where the rubber’s response is driven into the so-called glassy region"

"This proves that the friction force under most normal circumstances is directly related to the internal friction of the rubber,"

So you post actually mentions the friction of the rubber against the other surfaces and says exactly what I was saying in the the way the rubber conforms to the rock (this involves internal friciton in the rubber) strongly effects the friciton of the rubber against the rock but the internal friciton of the rubber is NOT THE ONLY/DOMINANT FRICTION IN THE SYSTEM. Sure it strongly effects how the external friciton works, but that's very different to what you were saying isn't it ! I am pretty sure that the dominant force of friciton (in terms of magnitude of force) in the system is that of the shoe against the rock not internally in the shoe and so far I can see nothing in your quote which disagrees with this, infact it seems to be agreeing to me. It certainly contradicts your post which I quoted above which says the internal rubber friciton is the frictional force in the system. Wrong it's one of the fricitonal forces in the system.

I can't be bothered with this anymore your an intelligent guy if you can not understand what I'm saying then I'll never get my point across will I ?

In reply to Woker:

Phew! I kind of lost track with this. However, I do think it is worthwhile pointing out that Woker must be on the right lines according to the simple principle that no matter what goes on internally within the rubber this cannot in itself be the source of an external force on the system as a whole (where this is the climber via his shoes). As we all know, internal forces cancel out in this sense. So, the friction between rock and shoe is in fact the only friction that matters in terms of keeping the climber on the rock.

This all takes me back to one of my favourite puzzles: boy on swing, feet off the ground. By twitching and thrutching he gets himself going then can increase his height by further twitching and thrutching, again without touching the ground. How is this possible? Taking moments about the point where the chains of the swing meet the frame, there is no net moment as all the forces are internal to the system as a whole (boy, chains, swing). And yet, gain height he does....

In reply to Woker:
the next paragraph of the paper I was citing is:
"The friction force between rubber and a rough ~hard! surface has two contributions commonly described as the adhesion and hysteretic components, respectively.1 The hysteretic component results from the internal friction of the rubber: during sliding the asperities of the rough substrate exert oscillating forces on the rubber surface, leading to cyclic deformations of the rubber, and to energy ‘‘dissipation’’ via the internal damping of the rubber. This contribution to the friction force will therefore have the same temperature dependence as that of the elastic modulus E( v) ~a bulk property!. The adhesion component is important only for clean and relative smooth surfaces."

I think that makes clear the point that you have to look at the internal deformations of the rubber for the origin of the friction of the rubber against the rock.

In reply to Woker:
... but the internal friciton of the rubber is NOT THE ONLY/DOMINANT FRICTION IN THE SYSTEM. Sure it strongly effects how the external friciton works, but that's very different to what you were saying isn't it !

No, it's exactly what I was saying. What you measure is the external friction of the boot against the rock, but the origin of that friction is in the internal damping of the rubber.

In reply to Woker:
I said:

and you said...

No. The two statements are completely compatible. For simplicity I focused on a piece of rubber as it went passed one spike on the rock. As the spike goes past, the rubber is first compressed, and then expanded. In reality the rock is covered with many spikes, so as the rubber slides any small piece of it is cyclically expanded and compressed. That's the meaning of cyclic in this context.

In reply to Richard J:

Richard,

Apologies if you've already explained this but surely there is a difference between static and dynamic friction. I can imagine that dynamic friction is determined by loss tangent - but static friction? I

In reply to Richard J:
The statements are generally compatible however thie bit which I am picking you upon is that you say the cause of friction in the system is the compression/decompression cycle. However even if it didn't compress and decompress at all the rubber would still have friction against the rock and infact the cause of the friction is not as you say the compression/decompression cycle but the movement of the rubber over the surface. The compression/decompression cycle determines the amount of friction which will be there whatever you do (even if you move the rubber over the surface faster than it can compress and decompress) and is not the "energy loss that causes friction" and you know it !!!

You also suggested that this compression and decompression is the reason your shoe stays upon the rock and that is not true. It's the friction of the rubber against the rock which keeps it there, and also the traction provided from the rubber moulding to the surface of the rock. Nothing you have said today or quoted today can change those facts at all.

Bye for now.....

In reply to Removed User:
That's a very fair point, but I did explain it earlier. The suggestion is that you never see true static friction for rubber, because for real surfaces, with roughness on very small lengthscales, even very small sliding velocities result in significant deformation frequencies. But this is a "for practical purposes" argument; in an ideal world there must be a true static friction coefficient arising from adhesion. But since this is so much smaller than the dynamic friction as soon as you stand on a hold your shoe will start to creep downwards...

In reply to Richard J:

Thanks Richard,

Does this also imply that we don't need the soles of our boots to be smooth?

In reply to Woker:
Obviously it is the friction of the rubber of the rock that keeps it on, what we're talking about is what the origin of that friction is, why rubber has such good frictional properties, why Fires and Stealth rubber are better than what went before, and, to get back to where this all started, why the friction of rubber depends on temperature so much. I've provided an answer to those questions, which may be incomplete, but it's the best one we've got. All you've done is simply assert that it's wrong.

In reply to Richard J:
No I have not simply assered that your wrong. In general as I pointed out you were correct and brought up some interesting points. Espeicaily the stuff about rubber deforming better or worse at differing temeratures and why that happens..

It seems to me that the reason cold grit could have better friction/grip on the rubber is that when cold the rubber still deforms but does so more slowly and so therefore the compression/decompression cycle also happens much slower, so therefore has better traction (rather than friciton) on the rock it has moulded around. Maybe the actual frictional qualities between the rock and the boot are nealy the same at most usual temperatures you would use the boot.

Anyway I think we are seeing the same points so let's call it a day.

In reply to Removed User:
I think the reason they are smooth is so they can better deform to the surface of the rock if they had patterns on the bottom the grooves would not often be incontact with anything..

In reply to John Gillott:

I'm no physicist, but surely the swing puzzle is all about resonance. Imagine that there are two extreme positions the boy can adopt on the swing, which give two different resting positions for the system (as measured by the angle of the chains, I'm not sure I follow your no net moment argument - there are certainly net resultant forces possible). All that happens is that these two stable positions are alternating at the resonant frequency of the system. Energy is supplied to acheive the change in position. No violation of conservation of energy or momentum. I can't really see the problem.

But then, as I say, I'm not a physicist.

In reply to Woker:

Fair enough.
If only we were still using clinkers and tricounis, we wouldn't have all this trouble.

In reply to Dave Garnett:

Try an analogy, which brings in friction again (oh dear!). If you are sat on a tray on the floor, you can hudge yourself accross the floor without touching the ground itself. However, it is my contention that if there was no friction (between the tray and the ground), this would not be possible, as every time you made use of the internal friction between yourself and the tray to move yourself, an equal and opoosite force would give the exact opposite momentum to the tray and the two would cancel out so you would get no where (try it on wet ice and you'll find it is true, you just thrash around getting nowhere).

Another way of putting the above analogy is that considering you and the tray as the whole styem, which is what we should do since we're trying to move them both accross the floor), there is no net force, the internal forces cancel. The same is true for the swing. Let's consider just the problem of gaining height once movement has already occured. The boy pushes on the chain say with a given force, this force has a moment about the point of contact with the frame and hence transfers angular momentum to the chain. But... the chain pushes back on the boy in just the same way in the opposite direction, giving him an angular momentum of equal magnitide, but in the opposite direction. Same goes for any use of friction by twitching on the seat etc.

I did try this on Midgets once and he struggled as far as I recall, so there's certainly something to puzzle over....

Woker: btw, your point about slower deformation in the cold was the argument made about climbing in the cold in an article by Adrian Berry (I think) on Planet-Climbing, but I can't find a link.

In reply to Richard J:

Maybe we should now ask the question: Do tricounis have better grip on rock in cool weather?

-------ARRRGGHH! Sorry.

In reply to John Gillott:

you're using the friction force (Ff=uN, unless you're dealing with polymers/gels/tissues which conveniently don't obey Amonton's law) between the tray/you and the floor to do work.


the interesting thing though is that without friction acting between the tray and the ground (eg. on wet ice you've got hydrodynamic lubrication - very low friction, but there is some) it takes only a very small force to start you moving. If there was actually zero friction, then theoretically you could just spit and that would be enough to move you in the opposite direction.


what you are neglecting though is that although there is no external forces doing work on the system (we'll neglect air pressure/drag and friction at the pulley as being negligible for the sake of argument), there is an exchange of energy within the system and this can create motion. In this classic problem (simple harmonic motion) you are basically exchanging potential energy (lost/gained as a result of changing height of the seat) for kinetic energy (lost/gained as the swing passes through the lowest and end points of it's arc of travel). You can show that for an arc half angle of theta, the velocity of the swing at the bottom of the arc is v=sqrt(2gr[1-cos(theta)]) where g=9.81m/s^2 and r is the radius of travel.

Other types of internal energy changes include e.g. springs (elastic), heat transfer, electric/magnetic fields. In reality there are losses in the system (friction, heat, etc) and so theta will decay with time - in a proper system you can use this information to determine what sort of lubrication mode is operating at the pivot point.

As for the climbing question - I reckon sweating in hot weather is the major factor. Depending on the rubber formulation, a temperature drop of 20degC (summer -> winter) might double the stiffness (modulus), so you could well notice a difference in edging, but that's nothing more than a guess really.

In reply to SimonG:

Yes, spitting or throwing objects would do on smooth ice... but the point of the analogy is that you don't do this (after all, the boy on the swing doesn't do something analogous).

Back to the swing. Errrrm, we aren't looking at SHM are we, a simple switch back and forth between kinetic and potential energy, but a gain in the overall energy of the system, which, for sure, at the top of the swing is all in the form potential energy, and at the bottom is all in the form of kinetic. Now, I agree, looking at the problem from the point of view of energy, clearly the boy's expenditure of energy is being transformed into kinetic and potential energy. The question is, how? Energy can me just dissipated as heat without being transferred to the system, and logically what is required is an explanation in terms of forces and moments if we want to explain what is going on. And from that point of view, looking at the system as a whole (boy, swing) it appears that there is no net external moment, ignoring gravity, which cancels out over a cycle.

In reply to John Gillott:
If the boy just sits still on the swing, we get a simple conversion of potential energy to kinetic energy. If the maximum angle is f, then in the usual small angle approximation the PE is mgl f, where l is the length of the swing. At the bottom of the swing this is all converted into kinetic energy, given by I w^2/2, where I is the moment of inertia and w is the angular velocity. So far so simple.

But the boy can change his moment of inertia by tucking his feet under his bum. So now he has two possible values of moment of inertia, a big one I1 with his legs stretched out, and a little one I2 with his feet tucked up.

So he starts with his legs stretched out. As he passes the bottom of his swing, he pulls them up. Angular momentum must be conserved, so his angular velocity increases by a factor I1/I2. When he gets to the top of his swing, he sticks his legs out again. As at this point he isn't moving this doesn't change anything.

Result - on every swing he increases his maximum speed by root(I1/I2). Where does the extra energy come from? The boy does work when he pulls his legs up.

In reply to Richard J:

Sorry - there shouldn't be a root in there.

In reply to Richard J:

I’m afraid not. Firstly, you haven’t answered the basic point: if there is a gain in the angular momentum of the system as a whole over a cycle, where is the net external moment? Secondly, to look at it in the way you pose it, there is no privileged stationary point at the top of the swing as far as the mechanics of this problem are concerned: the boy is only stationary for an ‘instance’. We’d better not get into that too deeply, but one thing should be clear: the boy cannot adjust himself in an ‘instance’, and the faster he tries to do it, the bigger will the effects be on the chain, seat, and the rest of his body. So, I conclude that no matter what he does, angular momentum is conserved at the top of the swing in exactly the same way as it is at the bottom of the swing. Finally, it is not even clear that the bodily movements you suggest will alter the moment of inertia in the way you imagine. The measurement that matters is the distance of the centre of mass from the point of contact between chains and frame. When the legs are thrown out they are thrown out roughly parallel to the seat, not in line with the chain. At the same time, to achieve this, the trunk is often thrown backwards. Similarly, when the legs are tucked up, the trunk is thrown forwards.

In reply to John Gillott:
I'm definitely not going to get involved in another lengthy physics debate. I've thrown out what I think is a plausible way of thinking of the swing problem, and if you want to do the complete analysis, taking into account the finite time it takes for legs to be moved, that's fine.

But I will just say that I think you misunderstand this general idea of internal forces cancelling out. It's obvious that a purely internal rearrangement of a system can result in an external force. Otherwise you wouldn't have to worry if a bomb exploded next to you. The theorem that you're thinking of proves that purely internal rearrangements of a system can't change its total momentum, or angular momentum, UNLESS an external force or torque is applied to the system. In your teatray problem, a force is applied to the system via friction. In the swing, the ground exerts a torque because as the swing swings the forces on the legs at the front and back of the swing are not in balance. This is very obvious on the swing in my garden, which isn't very well anchored. You can see the legs of the swing lifting up on one side as the swing goes up, then sinking into the earth as the swing goes the other way. Conclusion - angular momentum is not conserved over the full cycle of the swing, because the ground exerts a torque. And how on earth could it be? The angular velocity is zero at the top of the swing, and a maximum at the bottom!

In reply to Richard J:

Correct me if I'm wrong, but I thought that in an explosion of a stationary object, if we sum up the momenta of all the bits we will get zero (on the understanding that momentum is a vector and not a scalar quantity), immediately after the explosion before gravity has got to work on them. Similarly, all the internal sheer, frictional and other forces within an object must add to zero if the object is to stay together.

The swing problem has it's quirks due to gravity, as you point out (no motion at the top, more at the bottom). The problem can be reposed then as follows: how is it possible for the boy, swing and chain to have more angular momentum in the lowest position after a period of time than at the beginning if no net external moment is operating? You're right about the frame being affected by the ground, but what has this got to do with the boy, seat and chain, taking moments about the point of contact with the frame? After all, any stresses and strains transmitted from the ground through the frame to the point of contact will have no moment...

In reply to John Gillott:

I have to agree with Adrian J. In any case, a free body diagram of the system will show that there are external forces acting (there must be since the swing is connected to the earth and you have to 'cut' the system somewhere to draw the fbd). There is tension in the chain/ropes (balancing the mass of the system and the motion of the swing), friction at the pivot (if it was a pulley eg, this would usually be modelled as a [angular] velocity dependent torque - this is where your net "external" torque comes in) and forces at the supports. Just because you take moments about the pivot point to eliminate some of these forces from that calculation doesn't mean it suddenly doesn't exist.

Secondly, you're modelling the boy as a particle (single centre of gravity/mass), but when the boy starts swinging legs/trunk this is no longer a valid assumption. It is perfectly possible to swing your legs up without moving the rest of your body - the body as a whole (but your legs in particular) has now changed potential energy.

I think I've had enough dynamics/kinematics for today!

In reply to John Gillott:
The reasone the swing moves higher with each swing is that at the top of the arch behind the boy the the boy pulls on the chains and leans back which if you imagine viewing from the side, tips the seat forward as compared to the direct line of the chains from where his hands touch the chains to the bottom of the seat. This motion helps accelerate the boy towards earth faster than gravity alone would allow as he his moving his body weight as compared to the angle of the chains.

Then at the top of the arch in front of him when he leans forward and tucks his legs in, the chains go past straight and kink slightly the other way as the boy leans forward and pushes on them. The extra bit of height gained from leaning back is put back in to the system and also as the boy leans forward as the chains are slightly kinked the other way he also accelerates towrads the earth slightly faster than gravity alone would allow.

So i think there are two actions going on :
- the greater acceleration towards earth achieved by moving body weight as compared to the angle of the chains

- the greater height which can be achieved by having slightly kinked chains if the kink kinks the seat out towards the direction of travel.

Well that's how I think it works in plain english anyway.

In reply to Woker & SimonG:

I should say btw that this is a genuine puzzle, to which I have my own best answer, which I will happliy give when everyone gets bored (just shout). Besides midgets I have also put it to a very eminent physicist who struggled for a while than gave the most amazing suggestion, which I put down to him wanting to change the subject (it was something about the variation in the strength of the gravitational field at the top and the bottom of the swing, something I imagine we can all agree would be so small in relation to the resulting effect even if it did make sense).

I am not modelling the system as a point Simon, but as an object with a centre of mass. You will remember the famous example of the skater spinning on ice: she can hardly be called a point mass either, and yet she is used to illustrate the conservation of angular momentum about a vertical axis running through her centre of mass: when she pulls her arms in she speeds up, when she throws them out she slows down.

Yes we need to cut the system somehow. What I am doing is cutting it to include the boy, the seat and the chain, all of which have mass and all of which function as a connected system (rather like the skater and her bodily parts) spinning around an axis. Trouble is Woker you can play around with the internal manipulations of the system as much as you like, I just insist that I can't possibly do the individual bits of maths and thankfully I don't have to because it is all connetced, like our skater again.

The only force suggested so far which might have a net moment is friction. But if I make the connection as point like as possible this will be very small indeed. Can this really account for the pretty rapid gain in angular momentum that is achieved. I doubt it....

In reply to John Gillott:
Ok then what about this idea :
pulling on the chains must shorten the pendulum to the point where you hands are and the angle changes of the chains. As the pendulum shortens and you pull back on the chains when at the top of the arch behind you. You also stick out your feet, if you did not you would not be able to pull on the chains.

What then happens is the angle from the top of the chain to your hands as compared to a straight line to the centre of the pendulum is now greater than the angle would be if the chains were straight, ie the section of chain above your hands is now higher up than it would be if you did not do this. So in effect it would be like having a shorter pendulum with your body mass attached where your hands. This greater angle means you will accelerate faster towards the centre of the pendulum as you have in effect achieved more height. As the length of the pendulum has no effect on speed at the centre of the pendulum as this is only effected by angle at the top of the pendulum and weight at the end of it, then you will now be travelling faster at the centre of the pendulum as when you shortened it by pulling back you also increased the angle/height.

The same principle works at the other end of the pendulum by leaning forwards and pushing on the chains. the reason you accelerate more when leaning back than push forward (but from experience I think you do accelerate by leaning forward) is that sticking you legs out allows you to get more leverage on the chains.

I personally think this has nothing to do with how ballerina's spin, as leaning back or forwards makes no difference to your weight on the seat, if you imagine the seat to be a round pole, instead of a flat piece of wood and as we all know you don't need a flat seat to be able to swing.

In reply to John Gillott:
The ground can and does exert a moment on the swing, because the force isn't directed straight up the poles. It's got a component in the plane of the ground. That's why you need to anchor the legs of a swing into the ground to stop it wobbling around.

You can't do problems with rotational motion just using a centre of mass approach, because the CoM is defined formally in terms of the sum of every mass in the system times its (vectorial) distance from an origin, while the moment of inertia is defined as the sum of the masses times the distances squared. That's why the boy can change his moment of inertia while keeping his centre of mass in the same place. (Think of three equal masses on a line from an origin. If they are all in one place, r from the origin, moment of inertia is 3 m r^2. Now move one mass h away from the central position, and one mass h towards the centre. The centre of mass is still in the same place, but the moment of inertia is now mr^2 + m(r+h)^2 + m(r-h)^2= 3 mr^2 + 2 m h^2 - i.e. it's increased.)

In reply to Woker:

Sorry Woker, aren't you just repeating yourself? (Not that we don't all do this to an extent!). You are doing all sorts of manipulations internally (a simpler one of course would be for the boy to switch to a standing position at some point, that would really shift his mass about). My argument is that these will all cancel out. In your e.g. the flip side is that the seat is shoved lower, as will be a part of your body. If you lever off the chains at the other end you will at the same time shove them back, then they will tug you as you stay in one connected system. Analagously, on parfectly smooth flat ice (an idealisation, I know), two people tied together can't move from A to B: one shoves off the other to move forward, the other goes back, then the connection between them tugs them both......

In reply to Richard J:

The ground has a moment on the frame, but how does it have one on the chain, seat, boy ststem? The force must be transmitted through the frame to the point of contact to have any effect on the system we're interested in. When it gets there it has no moment, no matter what direction it is 'pointing' in, assuming an idealised point contact, which I maintain is a valid approximation for this puzzle.

I take your second point, and I should be more precise, but I don't see its relevance. The angular momentum of the system is the sum (integral in the limit) of lots of masses times distances squared, no? (with due allowance for directional aspects). Over a cycle, shouldn't this sum be conserved if there is no net moment operating?

In reply to John Gillott:

Make that masses times distances squared times speed of rotation of course.

In reply to John Gillott:
I think the fact that a body can alter its moment of inertia without changing its centre of mass is key to understanding the swing problem. But let me give you another problem which raises the same issues in even purer form. How come, when you drop a cat, it always lands feet first? If you hold the unfortunate animal upside down it has no angular momentum. When it is in free fall, it's difficult to see how any torque can act on it. And yet the clever beasts can still manage to twist themselves round to land on their feet.

This problem has a real solution, which does involve the cat manipulating its moment of inertia, but I will leave it for others to solve...

In reply to Richard J:
not all that difficult to do humans do pike dives rather easily also..... (ie body changes from horizontal to vertical position in the air)

In reply to Richard J:

Well, I must say, I have just cheated and looked it up. The point seems to be that the cat must have some spin to start with, and what is more its angular momentum is conserved during the fall. What is does is to change its moment of inertia so as to alter the speed of its spin (all the while conserving angular momentum), so that when it hits the ground it is oriented in the right way. A neat bit of intuitive behaviour, but I must say it rather confirms that the swing puzzle is still in need of a solution.....

In reply to John Gillott:
the truth
"If a forcing function is applied to a swing at the natural frequency of the swing it will resonate. The amplitude of the swing will increase during each back and forth cycle. The forcing function can be provided by a second person pushing on the swing. In this case even a small child can make a large adult swing by pushing in sync with the swing's back and forth cycle. The forcing function can also be provided by the person in the swing. In this case the person in the swing shifts her center of mass very slightly by changing the position of her legs or torso. This creates a slight pushing force which makes the swing go higher and higher. It takes a very small force but it has to be timed perfectly. "

see web page :
http://www.intuitor.com/resonance/swings.html

this is more in line with what i was saying than what you were saying isn't it John ?

In reply to Woker:

Woker, Oh it is, but I really don't buy it, and nor have the physicists I've put it to. The external bit is of course right, but the internal little force argument just ignores the equal and opposite force the boy exerts on the swing, to which the boy is forever wed. But I will dig around myself now since we've opened up searching the www!

In reply to John Gillott:
yeah yeah whatever ))

It doesn't matter what the chains are attached to on the swing. Attach them to a passage in a cliff if you like you will still be able to swing.

In reply to John Gillott:
To help your search, the technical term for situations like the boy on the swing is a "parametric oscillator". It's called that because it is an oscillator which is driven not by an external force, but by periodically changing one of the parameters (that would normally be constant), like the length of the pendulum, or, equivalently, the moment of inertia.

In reply to John Gillott:
i think you may have fogotton to consider gravity properly in this problem. when at the top of the archs gravity acts down not in line with the chains so pulling on the chains has a very different net effect to doing so at the resting point of the pendulem.

In reply to Woker and Richard J:

The history of this (for me) btw was when a teacher at school gave us the explanation given on the webpage, then admitted it didn't really work, as is sometimes the case with these things. At a leuisurely pace I will see if any of your suggestions turns up an answer, then I will have you all eating humble pie for the weekend )

In reply to John Gillott:
Two things you should consider. I know you are very keen on all the forces being balanced, but they are not. At the bottom of the swing, the boy is accelerating by virtue of the fact that he's going in a circle. There must therefore be a net force acting, which arises because the tension in the chain is greater than gravity. Likewise, following up what Woker said, if your just considering boy, seat and chain as the system, then of course there's an external torque acting - it comes from gravity. Specifically, when the swing is at an angle f to the vertical, there's a component mg sin(f ) of the gravitational force perpendicular to the chain, resulting in a torque mgl sin(f ) .

By the way, I stand by my original explanation. Good luck with your search!

In reply to Richard J:

Eh? Bloody hell Richard, I know all that. Of course there is a net force acting on the boy, swing and chain during circular movement. Thing is, as any A level student knows, acceleration during circular motion is towards the centre of the circle, thus so is the force on the system, and of course a force through the centre has no moment about that point. Similarly I'd pointed out above that the swing is a little different from the skater on ice problem since gravity has a (varying) moment on the system as it goes back and forth. That's why, ignoring air resistance and any other friction, the system performs Simple Harmonic Motion rather than simple revolving around and around. My question is, how can it be made to perform a motion with an increasing amplitude with no apparent net external moment over a cycle (since gravity acts first one way and then the other in a way that perfectly cancels out? The forces that I reckon do cancel out are all the little internal ones: boy pushes seat, seat pushes boy etc. Newton's Second Law IIRC.

I do btw have an answer to this (which may or may not be right, I'm hoping for enlightenment on this), and a quick skim shows that it has something to do with your oscillator, but perhaps not in the way you suspect. As a clue I will suggest to Woker that in a solid arch he will have a harder job gaining height than you will Richard on your garden swing.

In reply to John Gillott:
very topical

http://www.brianmung.com/blainegame.htm

In reply to John Gillott:

Richard J (sorry for calling you Adrian before!) is absolutely correct, so if your answer doesn't say the same thing then you should consider yourself enlightened!

You're only considering motion and forces in the normal direction (ie. towards the centre of curvature of the motion - the pivot in the swing problem) if we consider an n-t coordinate system which is perfect for describing cicular motion. Then your acceleration in the normal direction is v^2/r, but there can also be a tangential acceleration, which in this case is essentially caused by gravity. In fact, there must be a tangential acceleration because the magnitude of the velocity vector is changing (it is zero at the top of each swing and a max at the bottom). The normal acceleration (acting through the pivot, so not generating a moment about it) only changes the direction of the velocity vector.

In reply to SimonG:

I responded to Richard's point about the bottom of the swing. At that point there is no tangential acceleration, so all forces go through the point of contact between chains and frame.

Yes of course there is tangential acceleration at all other points, and the only thing causing that, I suggest, is gravity (we can bring in all sorts of frictions between boy and seat of course, but these are just the little things that cancel. If it help,s get rid of the swing and have the boy suspended like a trapeze artist with safety wires on but without the bar). Now, yawn, gravity acts first one way, then the other, then the other, etc etc, harmonic motion, fixed amplitude.......

In reply to John Gillott:
Sorry - it's not always obvious what's obvious, if you see what I mean.

I still slightly suspect you of beginning to fall victim to a common affliction caused by too much staring at Newton's third law of motion, which leads to a growing conviction that no motion of any kind can ever be possible at all...

In reply to Richard J:

I wonder: has there ever been such a mindboggling scientific discussion on RT before?! It would be helpful when it is all resolved if someone could write this up in just 4 or 5 very well written sentences for the layman.

In reply to Gordon Stainforth:

You must have missed John Cox's two envelopes puzzle if you think this is bad. Now we really should run that one again. John, the floor is yours..

In reply to Gordon Stainforth:

Think levers not forces. The legs extend at the top of the swing to increase force, or the weight on the downwards swing. When the swing is past the bottom the legs are retracted to minimise the retarding force and only extended to provide more weight at the top again.

IS this thread still going?

I'd given up on it a long time since!!

In reply to John Gillott:

John, your refusal to listen to Richard's fairly clear explanations is starting to annoy me.


Er, no. This is only true of circular motion at a constant angular velocity, which is patently not the case on a swing.

Although that is slightly beside the point. To come on to:


It only perfectly cancels out if the boy sits still. If he moves so that his centre of mass is further out on the downswing than on the upswing (Richard has explained why this is possible) then the gravitational torque will be greater during the downswings than the upswings and the amplitude of the motion will increase.

Oh, and there is obviously no net external moment over a cycle, otherwise the boy would spin around the pivot at an ever-increasing velocity. There _is_ a net external moment over each half-cycle, as I have just explained.

Looking at it from an energy rather than angular momentum point of view, the energy required for swinging (as Richard has also pointed out) comes from the fact that the boy must move his centre of mass inward against the centrifugal force at the bottom of the swing (so doing work) while he moves it out at the top of the swing where there is no centrifugal force acting and so no energy returned (and don't get me started on whether centrifugal force is a "real" force).

In reply to darkinbad:

I do apologise, he said a few things early on which were plainly off the mark (such as the idea that the reaction betwen ground and frame could generate a moment on the swing), so maybe I didn't pay enough attention later on. I shall be delighted if he has pointed me in the right direction since it is a little puzzle I've entertained myself with over the years without really getting the books out to crack. I am avoiding using energy since I was always taught as a lad that it could be done without in mechanics problems, and it is easy to be led astray using energy and kind of read things backwards. After all, we know the boy is trying to pump energy into the system, just as two people tied together on flat smooth ice are expending energy trying to get from A to B.

I'm all ears, and ready to look at a few old books on the shelf. If the answer is a simple as you say I shall laugh at myself and offer apologies all round.

In reply to John Gillott:

OK, laughing, smiling and aplogising, especially to Richard

www.tam.cornell.edu/~jayant/submitCDC03.pdf

I really must look up possible solutions to classic problems.

In reply to John Gillott:

But where is the tangential force that increases the boy's linear momentum when he stands up at the base of the swing?

In reply to Anonymous:

Now now Plank, don't go causing trouble, I'm in enough already. However, since you ask, I will soon send you a response which shows that your solution to this is plainly wrong