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

Simple. Just draw a free-body diagram on the cup. Static friction (paper
on cup, parallel to surface) is one of the forces on the cup in the
direction of motion. Thus, it must do positive work.

Forum for Physics Educators <> on Thursday, March
10, 2005 at 9:55 AM -0500 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.


Bob at PC
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