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

[Brian Whatcott <betwys1@sbcglobal.net>]

At 03:22 PM 3/14/2007, Bob, you 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?
>
> 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. What is the energy transfer (or whatever your
> favorite terminology) mechanism for the static friction example above?
>
> Bob at PC

A force that does not meet with movement does no work.
So the tailgate hinge does no work, the cabin door does no work
and the floor carpet does no work.

But wait a moment. The truck is accelerating, so something
 is doing work, I suppose. What could it be?

The engine is doing work on the prop shaft.
The prop shaft is doing work on the differential.
The differential is doing work on the wheel/tire.
But the contact patch is not in motion, so it's hard to say that
the tire is doing work on the road. But in reacting the applied
torque, the wheel is doing work on something, I imagine.
 Ah yes: the wheel though fixed (instantaneously?)  at
the road surface, moves the hub with a certain force.
Then that must be what is doing productive work then....?



Brian Whatcott    Altus OK    Eureka!