Coefficient of friction lies between 0.1 and 1.5

1. Aug 28, 2014

Yashbhatt

My textbook says that the value of the coefficient of friction lies between 0.1 and 1.5. But I see no reason why it can't be 2,3,4,5 etc. One just needs to apply a force greater enough to move an object if the coefficient of friction has a greater value. What is the actual thing about it?

2. Aug 28, 2014

jz92wjaz

In theory, it is possible. High coefficients may not be meaningful because the materials are likely to be deformed or destroyed instead of sliding.

3. Aug 28, 2014

Matterwave

Imagine you have a bar of metal welded onto a table made of metal. The "coefficient of static friction" there must be enormous! But at that point, perhaps it's better to model the situation in a different way.

4. Aug 30, 2014

gsal

I guess it could be 2 or 3...the actual thing about it is that, so far, experiments between a surface and an object on top of it have yielded values within that range (rubber to rubber as large as 2).

Then again, for as long as friction raises from the (inter-atomic) forces between the molecules of the bottom surface and those of the object on top of it, there probably is a limit to friction coefficients.

5. Aug 30, 2014

jbriggs444

For the "coefficient of static friction" to be meaningful there must be a linear or at least an approximately affine relationship between normal force and friction. I do not see one in the case of a weld. Though that may just be because I'm not familiar with the mechanics of weld failure.

One could set up a pair of greased meshed gears and achieve an arbitrarily high "coefficient of static friction" by choosing the angle of the gear teeth.

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Last edited: Aug 30, 2014
6. Aug 30, 2014

AlephZero

Coulomb's simple model of friction is just a piece of convenient mathematics, that works fairly well in some real-world situations, and is simple enough to use in hand calculations.

It doesn't contain any physics at all. It seems to be a common misunderstanding that it is some kind of universal physical "law".

If the loads are sufficiently high that the objects deform before there is any macroscopic "slipping" motion, Coulomb's model is usually not very accurate. That's one reason why it doesn't make much sense to talk about "static friction" in Coulomb's sense with friction coefficients greater than about 1.0. You really need to look at the stress and strain distributions over the contact area of the two flexible bodies, not just the total values of "normal" and "tangential" force. But you can't do that without computer simulations, and a lot more understanding of mechanics than you need to solve high-school-level textbook problems using Coulomb friction.

7. Aug 30, 2014

Matterwave

You are right. I don't believe a weld can be described by a coefficient of static friction. I guess I didn't think it through!

8. Sep 1, 2014

Murtuza Tipu

Can you explain me how rubber to rubber be large as 2.
How to calculate it ?