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

Perhaps my view is simplistic but it seems to me that one can answer the
question by first asking what happens to the energy of the coffee cup? If it
is maintained at a constant velocity on a level surface, it neither gains nor
loses kinetic or gravitational potential energy. If there is no rubbing of
the paper across the coffee cup, there is no gain in thermal energy.

On the other hand, if the cup accelerates then the paper is doing work on the
cup, just as your hand would have if the cup had been sitting on your hand.

Where does the paper get the energy that ultimately accelerates the cup and
itself (if that occurs) and to add thermal energy to itself and the desktop
by rubbing? From your hand.

Jim

On Thursday 10 March 2005 09:55, 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

--
James R. Frysinger
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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