Re: Work done by Friction

[Roger Haar <haar@physics.Arizona.EDU>]

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 but the interactiom between the block and the desk
involves the dissipation of mechanical energy into thermal energy (the evil
"HEAT" word).  Thus from the ground's frame, the energy transfers from the desk
(and truck) is the sum of the block's increased KE and the new thermal energy.
But in the truck's frame, the block loses  KE and it is that energy that
becomes thermal energy.

        If one block pushs another, we speak of the one block dong work on the
other.  Ususally we omitted mentioning the nature of the contact force. If this
force was transmitted by way of a spring, the compression of ( and thus enery
stored)  in of the spring does not need to be considered to find the change of
KE of the pushed block.  (Although it must for the pushing block.) If one block
sits on another, one can omitted the discussion of nature contact (frictional)
force to find the change in the KE.  It is just that other things are also
going on.

Thanks
Roger Haar
U of AZ