Date: Thu Aug 7 22:09:36 2008

Author: Steve Wonnell

Subject: Re: Friction lab, depends on velocity?


Hi Tom,

Professor Mark Robbins in our Department is something of an expert
in friction, and he has been educating us that yes, indeed, the
simple treatment of friction presented in elementary physics
textbooks is indeed only a very rough approximation that does
not always apply. There is a recent article addressing specifically
the dependence of velocity that may better answer your question.
See "Increase in friction force with sliding speed" by Rod Cross
in the September 2005 issue of American Journal of Physics
(volume 73, pg. 812, 2005).

See also:
J. Ringlein and M. O. Robbins, Understanding and illustrating the
atomic origins of friction, Am. J. Phys. 72, 884 891 2004.

Java simulations of frictional motion can be found here:

-- Steve W.

On Thu, 7 Aug 2008, Carter, Tom wrote:

> Folks,
> I was getting a lab ready for the fall term and I wanted to show the
> students that the coefficient of kinetic friction doesn't depend on the
> velocity of the object. That is, as long as you're moving at a
> constant velocity, it doesn't matter what that velocity is, the force of
> friction is the same. (This ignores air drag.)
> The problem is that when I tried the lab out, it turned out that the
> coefficient _DID_ depend on velocity, contrary to every intro physics
> book I've read!! It's a small effect, around 10%, but it's obvious and
> easily reproducible. I need some help!
> Here's what I did....I attached a vernier force probe to a short 2X4
> wooden block and then pulled it along a Pasco aluminium lab track at a
> relatively constant velocity. (I used a motion detector to check my
> velocity rate.) Okay, this is blank wood on aluminum so there aren't any
> weird trend effects or fluids involved. But I need about 10% MORE
> force to pull at a constant 0.4 m/sec than at 0.2 m/sec.
> The speed very low so there shouldn't be significant air drag. I
> tried increasing the mass of the system by putting a brass mass on the
> block, but the effect was still there.
> So two questions:
> 1) Anyone want to explain this? Are all the physics books (and
> my lectures for the last 8 years) wrong?
> 2) Can someone propose a demo that WON'T so this effect? I'm
> hunting around and was thinking of putting felt on the bottom of the
> block, but I was hoping for some sage advice.
> Thanks.
> Tom
> ------------------------------------------
> Dr. Tom Carter
> Physics
> College of DuPage
> (o) 630-942-3346