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Re: [Phys-l] Work done by Static Friction



Yes, I should have been clearer on the reason for asking the question. I
agree with what you have stated, and also with Brian and Dan. The
context of my question is the blanket statement in many texts (Serway is
the one I'm currently using) that static friction can do no work because
there is no displacement involved. As pointed out by you and Dan, a
person on the ground sees a displacement and work appears to be done by
static friction.

But Brian makes a point that might be what the texts are alluding to. He
shifted the question to what the stationary road is doing to the tires.
Here we have a static frictional force (with no displacement of road or
bottom of the tire) being applied - and an associated acceleration of
the truck. No one would argue in this case that the road was doing work
that resulted in the KE of the truck - the engine is ultimately the
source of the motion.

The responses so far confirm my conclusion that the blanket statement in
the texts is simply wrong - that they were only considering a narrow set
of examples when formulating the statement.


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Wednesday, March 14, 2007 9:09 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Work done by Static Friction

On 03/14/2007 05:22 PM, LaMontagne, Bob wrote:
A crate sits on the bed of a flat-bed truck. The truck is initially
at
rest and then moderately accelerates. The crate does not slide, but
simply follows the motion of the truck. Did the force of static
friction
that accelerates the crate do any work?

Joe, an observer in the lab frame, says yes.
Moe, an observer comoving with the truck, says no.

Both are correct.

What is the energy transfer (or whatever your
favorite terminology) mechanism for the static friction example
above?

In Joe's frame, the key part of the mechanism is "static friction".
To say it in more detail:
-- static friction explains the force,
-- force dot dx explains the work
-- work explains the change in KE in accordance with the relevant
work/KE theorem.

In Moe's frame, force dot dx is zero. No work. No problem.

These answers are so obvious that I worry that I have missed the
point of the question ... and I wonder why the question was asked.

I ask this because when a block is decelerated by kinetic friction,
the
KE of the block decreases and the internal energy of the surfaces
involved increases.

Context like that is usually very helpful, but in this
case I'm still not seeing the point. Now we have four
cases: Moe static, Moe sliding, Joe static, and Joe
sliding.

Here are my attempts to figure out the point of the question:

1) In /general/ you have to be careful about applying the laws
of physics in an accelerated reference frame, but in this
/particular/ case the acceleration-related contributions are
zero, so I don't think that's the problem.

2) As always, you can get into trouble if you start out in Joe's
frame, transform into Moe's frame, calculate the delta KE,
and attempt to use that result as if it were valid in Joe's
frame. That's not even an acceleration-related problem;
that's a velocity-related problem. Energy is a scalar in
3-space but not in 4-space. That is, energy is invariant
w.r.t spatial rotations but not invariant w.r.t boosts.

I call this the "bat effect". In the CM frame, a bat
hitting a ball does no work. However in the batter's
frame, and in the usual spectator's frame, the bat does
considerable work on the ball.
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