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

This is a different example conceptually. The static friction
against the foot is truly static. The force of static friction
is numerically equal to the mass of the person times the
acceleration of the Center of Mass of the person. The energy is
generated internally from the leg muscles.

When dragging the coffee cup, the ultimate source of the energy
seems more external, i.e., the muscles of the person pulling the

I guess the connection for our purposes is that in both cases the
static friction supplies the force (and the kinematics by use of
Newton's 2nd Law) but is not the source of the energy.

The comfort I am taking from everyone's responses is that it
appears that the general rules that "static friction never does
work" and "the Normal force never does work" are not actual
truisms - there are many counterexamples to the rules.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-] On Behalf Of QUIST, OREN
Sent: Thursday, March 10, 2005 12:54 PM
Subject: Re: Quick question on static frictionHi All

I think everyone is making this too complicated.

How about a person walking across a room, or a car starting to
Isn't it static friction that produces the forward force that
results in
the velocity (K.E.)?

Oren Quist, South Dakota State U.

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-] On
Behalf Of John S. Denker
Sent: Thursday, March 10, 2005 11:28 AM
Subject: Re: Quick question on static frictionHi All

On 03/10/05 09:55, rlamont wrote:

If I place a coffee cup on a sheet of paper and pull on the
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
Scientists and Engineers) implies that no work is done by the
static frictional force because there is no actual
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.

I agree with what several others have said.

Let me thrown in an additional cent or two:

1) Analyzing this situation in the accelerating frame
of the paper is not a good way to go. I'm not saying
that it can't be done ... I'm just saying it's beyond
the scope of the course. It converts a HS kinematic
problem into a general relativity problem ... probably
not what you wanted.

This is conceptually slightly -- very slightly --
tricky, since you want to use the accelerated frame long
enough to observe that the fricion is indeed _static_
in that frame. Then you want to use an unaccelerated
frame for calculating the energy budget.

Tangentially relevant reference: "What makes the car go?"

2) It's not wrong to say that the hand does work on
the paper ... but this does not exclude the possibility
that the paper is doing work on the cup.

Here is an important conceptual point: energy obeys
a local conservation law, which is conveniently
expressed in terms of a conserved flow.

In this case, energy flows from the hand to the paper,
and thence most of it flows into the accelerating cup.
(Some of it is used to accelerate the paper itself,
and presumably some of it is dissipated at the
paper/table interface.)

3) The following has a wide applicability, not limited
to just friction, and may be a contributing issue here:
Many people have learned the theorem that a force of
constraint does no work. But that's wrong; the real
theorem says that a *stationary* constraint does no
work. If you have a moving constraint, all bets are
off. For example, suppose we have some water that is
constrained by the walls of a ladle. For a nonmoving
ladle, these are clearly conservative forces i.e.
the derivative of a potential. But if I'm allowed
move the ladle, I can easily schlep water from a
lower bucket to a higher bucket, doing as much work
as I like.

This may sound like a triviality; you may be wondering
how anybody could be so dumb as to mis-state that
theorem ... but I've seen it done. I've seen Ivy
League professors get flummoxed by this.
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