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Re: Friction



John,

In terms of explanations, I agree with Rick.

In terms of the demonstration, have you tried covering both
sides with felt? This will give both edges a more consistent
coefficient of friction.

We used blocks of wood in our lab on friction and found that the
surface of the blocks caused all sorts of jumpy results. It works
out much nicer when we covered them with felt.

tom

Rick Tarara wrote:

First: The 'independent of surface area' empirical law isn't strictly true
(try making the area VERY small). Certainly one can show that the
frictional force does not scale in any significant way with the surface area
as most naive expectations would have it do.

Second: Almost certainly the large and small surfaces are not identical in
flatness, polish, etc. such that the requirement that the coefficients of
friction are identical is only approximately true in your case.

Rick

-----Original Message-----
From: John DaCorte <jdacorte@mdihs.u98.k12.me.us>
To: PHYS-L List <phys-l@atlantis.uwf.edu>
Date: Thursday, October 29, 1998 9:28 AM
Subject: Friction

Hi,

I was wondering if someone could help me with a demonstration I have had
trouble with. I place a block of wood on a tilted desk and tilt it to
the angle that just makes the block move and secure the desk. Then I put
the block on another edge (with a different surface area) to show that the
frictional force and the coefficient of friction do not change with
surface area. The problem is that the block always stays put on the side
with the large surface area and slides on the side with the small surface
area. Any explanations? Thanks in adavance for any help.

John DaCorte