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

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Something is wrong here;  either I don't understand the question or S.
has nodded. [Bonus dormitat Serwayus.]

What accelerates the cup?  A force.  What object applies the force on
the cup?  The paper.  Does the  positional relationship between the
paper and cup change (slip)?  If not, it's static friction.  If so, it's
ordinary friction.  In either case by definition work is done by the
paper thru friction.  Once it's accelerated, i.e. constant motion, work
is being done by the paper on itself and the table.  (ordinary
friction).  This is an opportunity for Mr. (Dr.?) Green.

Perhaps S. is thinking of the case where one applies a force, e.g. pull
on paper, but no movement. (we ignore work done stretching.)  In this
case static friction does no work.

bc, who often sleeps.

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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