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Re: Work done by Friction



On Tue, 21 Jul 1998, Roger Haar wrote:

William Beaty wrote:
We agree that work is done in speeding up an object.

It's wrong to say that work is done in slowing that object down?

In what frame of reference? If a block comes to rest relative to my desk,
I can pick a frame of reference in which the block is speeding up. For
example, the desk is in a truck that has just accelerated up to cruising speed,
and I choose the ground as my rest frame. In this case the block speeds up and
ths it's K.E. increases.

Is "work done upon a block" still a useful concept when in a non-intertial
frame? We then have pseudoforces doing crazy things, or we can see
inertia (and therefor mass) misbehaving, acceleration losing meaning, etc.
Maybe I should have first specified "As far as basic Newtonian physics
concepts are concerned, we agree that from an inertial frame, work is done
in speeding up a block."

If we can't agree that work is done when speeding up a block, then
friction issues are secondary.


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