Date: Fri Aug 8 12:24:31 2008

Author: Jerry DiMarco

Subject: Re: Friction lab, depends on velocity?


Well, the Cross article in AJP did say that elastically hard
materials accelerated uniformly, and their coefficient of friction was
essentially constant. Is it possible the block is tipping forward
imperceptibly when you pull it? It would be interesting to see if the
results are different for a long block, or if the height of the attachment
point makes a difference...


At 8/7/2008 05:29 PM, you wrote:
> 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
>College of DuPage
>(o) 630-942-3346


Jerry DiMarco
Manager of Lecture Demonstrations and Instructional Labs
Montana State Univ., Physics Dept.
Bozeman, MT

Our Motto: "Find your inner demo."