[Phys-L] Re: Quick question on static frictionHi All

[Michael Edmiston <edmiston@BLUFFTON.EDU>]

The idea that "no work is done by the static frictional force because
there is no actual displacement of the cup relative to the paper" seems
the bogus idea to me.  Since when does work require displacement between
the object being worked on and the object causing the force?  As Bob
Lamont pointed out, if you push the cup with your hand, there is no
actual displacement of the cup relative to your hand, yet everyone would
agree that the force of your hand is doing work on the cup if it is
causing a change in energy of cup.

In the free-body-diagram of the cup (which is on top of the paper, which
is sliding on the table) I would draw three forces... (1) the weight,
(2) the normal force, (3) the static friction force.  However, I would
probably not label this third force as static friction.  Rather, I would
label force-3 as F(cp), which I read as "the force on the cup caused by
the paper."

> From the FBD it is clear that as long as F(cp) exists, it is the only
horizontal force on the cup, and therefore it *will* accelerate the cup
and increase the cup's kinetic energy.

If the cup/paper system is being pulled at constant velocity, then it is
clear that F(cp) is zero, and that also means there is no static
friction.

One reason I would label force-3 as F(cp) rather than static friction is
I agree with Roger Haar that the typicall F(sf) <= u(s)N is problematic.
I do not want students to think of force-3 as uN.  Rather, I want them
to think of it as the force exerted on the cup by the paper, and since
it requires friction between the paper and the cup, the maximum value of
F(cp) is u(s)N.


Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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