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

Hi,

This problem requires some care. There is no
displacement in the (accelerating) frame of the
paper and cup. There is a displacement in the
table's frame. I think that the static friction
is doing work on the cup.

An additional complication is the bothersome
inequality symbol. We do not have an equation for
determining the magnitude of the static frictional
force other than it is exactly large enough to
cause the acceleration of the cup.

A similar problem is a v-belt on a pair of
pulleys, one driving, one driven. The belt does
not slip on the pulleys, so static friction is the
force of the belt on the driven pulley which
causing the pulley to rotate. It is easier to just
do energy into the driving side and out the driven
side but the details are a mess. Even how the
belt transmits the energy is subtle.

One can make the claim that there is no work done
by a normal or contact force, because there is no
displacement (for example between me and the
floor) Consider the case in which the floor is
the floor of an elevator (after the initial
startup and before the stop) The elevtor is
moving at a constant velocity relative to the
Earth. My potential energy is increasing. Does
the normal force do work on me? I can easily do
the integration or just
delta PE = W = mgh

***********************************************************************
rlamont wrote:

Hi All,

I have a quick question related to static friction that came up
in class today - and I just don't have time today to search the
archives - so forgive me if I'm traveling an old path here.

If I place a coffee cup on a sheet of paper and pull on the paper
gently enough to accelerate the coffee cup without it slipping on
the paper, has the force of static friction done work on the
coffee cup? One of the homework problems in Serway (Physics for
Scientists and Engineers) implies that no work is done by the
static frictional force because there is no actual displacement
of the cup relative to the paper. I don't find that very
satisfying because the cup still gains kinetic energy and the
only force acting horizontally on it is the static friction.

Help!

Bob at PC
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