Date: Thu Aug 7 23:17:40 2008

Author: Bernard Cleyet

Subject: Re: Friction lab, depends on velocity?


10% is no change in intro labs.

bc only 80% serious.


p.s. when I taught lab at Ventura College a coupala students were
careful enuff to detect non-isochronism -- they hadn't reached
pendulua in the course, so I treated it la PER (before I knew it.)
it was wonderful and taught me something, as always before it was
lost in the noise.

On 2008, Aug 07, , at 15:29, 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