Re: Work/Energy Theorem

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

Al Bachman's comments on the seldom recognized importance of internal work
are well-taken.  I have only one comment. 

> 2. For a DEFORMABLE body, the internal 3rd law pairs of forces CAN DO WORK.
> The internal work associated with those forces can cause the center of mass
> to accelerate, IF there is an external constraint.

Although it certainly is arguably a semantic issue, I wouldn't say that it
is the internal work that "causes" the CM to accerelate; only the
unbalanced external constraint force can do that.  It IS the case that the
energy that shows up in the bulk CM motion is extracted from the internal
energy; it is NOT necessary for an external agent to do work to accelerate
an object.  It IS, however, necessary for an external agent to provide
impulse to accelerate an object.

> 4. The above example emphasizes that the external force does NOT have
> to be associated with the SOURCE of the energy. That energy may actually
> come from the work done by inernal forces acting over the DEFORMATIONS
> of the body.

Yes.  And this is a point well worth making as it begins an enlightening
trip over the bridge from mechanics to thermodynamics. 

John
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