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



Easily contentious! I am uneasy about your chosen path, which probably was not meant literally. The energy from the engine appears not in the wheels, but in the entire vehicle. There are hundreds of internal action-reation pairs that have work associated with them that appears as KE of all the various parts of the vehicle, just as in pushing ourselves away from a wall the work from the internal contraction of our muscles ends up as our gross kinetic energy.

Bob at PC


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Brian Whatcott
Sent: Sat 3/17/2007 2:02 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Work done by Static Friction

I was bemused by all the consternation, the delicate tap-dancing
and the curious mix of self-congratulation for doing physics that
I read in this thread, when upon reading Rick's note below I
realized the issue.
This seems to be a difficulty arising from the use of a particular
model, in a case which "does not simply follow the rule".

Sure static friction can allow a part to speed up on a truck bed. And a
bolt and nut can allow another truck part to speed up with the rest
of the bits.
And sure, these fixings and forces of adhesion can apply a force.
Moreover, if there is a structural member involved, the one end of it can
apply a force to the other end of that steel strip.

But I don't typically think of half a strip of steel doing work on its
other half. There is no relative movement to satisfy the conditions
for the "work" model.

But hang on a darn minute: wasn't I writing in the last day or two that
an engine does work on a drive shaft? And so on, as far as the wheel/tire?
Would it be any more satisfying if I said instead that the engine passes
energy to the transmission, drive shaft, differential etc.?
That energy flows through these components? Like I sometimes talk
about heat flowing though a conductive strip? That's Jim territory,
and I easily see it is contentious.

Brian Whatcott

At 06:52 AM 3/16/2007, you wrote:
No time to look this up (64 papers to grade), but I seem to recall that one
can preserve the Work/Energy theorem in this case if one looks at what is
happening microscopically? Maybe akin to the fact that any horizontal
surface must deform slightly to hold up a mass placed on that surface? Just
wondering.

Rick [Who really doesn't have a problem with the fixed observer saying that
static friction is the force that causes the displacement of the crate.]

----- Original Message -----
From: <carmelo@pacific.net.sg>

Some physicists proposed that 'static friction' to be
named "shear adhesion" instead. You may consider transfer of "Coulomb
energy" from the microscopic view?

Alphonsus


Brian Whatcott Altus OK Eureka!


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