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

[rlamont <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

I guess that's the source of my uneasiness with this. I agree
with you that my hand is the ultimate "source" of the kinetic
energy of the cup. Serway makes a general statement that static
friction does no work. But I'm finding it hard to see why the
situation where I accelerate the the cup directly with my hand,
my hand is doing work - but if I use the sheet of paper as an
intermediary to accelerate the cup, the static frictional force
is not doing work. How come my hand in contact with the cup does
work, but the paper (via static friction) does no work? Suppose I
grab the cup handle between my two fingers and only use static
friction to pull the cup? Is it me doing the work or the static
friction force?

I always find it hard to apply hard and fast "rules" to
situations with macroscopic forces - the whole concept of a force
seems to become ill defined.

Bob at PC

> -----Original Message-----
> From: Forum for Physics Educators [mailto:PHYS-
> L@list1.ucc.nau.edu] On Behalf Of James Frysinger
> Sent: Thursday, March 10, 2005 10:07 AM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: 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
> Lifetime Certified Advanced Metrication Specialist
> Senior Member, IEEE
>
> http://www.cofc.edu/~frysingj
> frysingerj@cofc.edu
> j.frysinger@ieee.org
>
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