# Friction and Cold Welds

1. Aug 24, 2004

### physicsisphirst

What is Friction?

Making contact amounts to the mutual repulsion of electrons on each body's surface. So is friction the lateral repulsion experienced as a result of surfaces not being smooth? In other words, the surfaces have roughness and so 'fit' into each other whereupon the electrons can push sideways against each other?

Halliday and Resnick book talk about placing machinist's polished gage blocks ?flat surface to flat surface in air? and that these will stick firmly to each other. Why? What is happening at the surface? How is this different from unpolished surfaces due to which you might get even more points of contact?

They also talk about 'cold welds' in which the metal surfaces at contact points get 'stuck' together. How is this possible unless the electron repulsion is somehow overcome? Or do surfaces of contact points somehow 'fit' together creating lateral resistance?

Ideas and explanations much appreciated. Thank you.

In friendship,

2. Aug 24, 2004

### K.J.Healey

I think when you have polished surfaces its less of a electron problem and more of an air suction problem. Just like suction cups on a windshield, or two pieces of glass on top of eachother (you cann feel the suction).

Its easier to experience with 2 pieces of glass, put water betweent them, and try to pull one off the other without sliding it.

OR you say unpolished surfaces could have more points of contact? I don't believe so, the flatter the two are, the more surface area will be in contact.

But friction is independent of surface area, its a function of the material and how hard something is pushing them together.

3. Aug 24, 2004

### check

So the 2 metal plates would not 'stick' together in a vacuum?

4. Aug 24, 2004

### physicsisphirst

very true. so in order for the vacuum to be maintained, there must be areas that are 'sealed off' from the outside air. this would require that at least some contact points were continuous and sufficiently non-porous as to not let air particles in.

i guess i was thinking of a tire :rofl: (i really wanted to use that emoticon)
the contact points even between smooth surfaces are few since only the parts that stick out actually get to make contact. however, i can see what you are saying makes sense because smoothing i guess makes more contact points by not allowing some of them to stick out that far.

that is an interesting question by check. would smooth surfaces not stick together in a vacuum. i guess not, because you wouldn't have the outside pressure pushing them together (even if you had dramatically reduces the pressure between the surfaces).

in friendship,

5. Aug 24, 2004

### K.J.Healey

tires are different because its more of a degradation of the material that leads to slippage. The surface area thing still holds true for the friction, but its more than pure friction that keeps a tire sticking. I guess you could say that the coefficient of static friction for the tire is stronger than the rubber's ability to keep solid under that force.

6. Aug 25, 2004

### LURCH

Truth is, nobody really knows yet what causes friciton, it is a suprising hole in our knowledge of the world around us. Some of the mechanisms suggested are the elasticity of two bjects deforming and recovering as their points of contact "brush past" one another, or the interaction of ellectrons along those surfaces. But for each mechanism there is a prediction arising from that mechanism which does not agree with observation.

7. Aug 25, 2004

### physicsisphirst

that is very interesting to hear!! i would have thought that friction would be one of those things that had been thoroughly studied and well understood, being as ubiquitous as it is. so now i don't feel too bad about asking this question :D

can you recall which sciam article it was?
i might try to pick it up on sciamonline.
thanks.

in friendship,