Re: [Phys-L] ?conservation of _internal_ energy

[Carl Mungan <mungan@usna.edu>]

> > How about this case: A guy on roller skates pushes off from a rigid wall. System = man. Any work done on the system? Does his internal energy change?
> >
> > Further, in the real world, the guy will get a bit hotter if he does this several times. Any heat transferred to or from him (on short timescales)?
>
> Howdy,
>
> Of course there has been work done on the guy! He pushes against the wall with a certain force. Therfore (Newton's 2nd Law) the wall MUST push against him with an equal and opposite force so work is being done on Him.


Work = integral of force dot product the displacement of the POINT OF APPLICATION of that force.

The wall (which is where the force is being applied) does not move. Ergo it has zero displacement.

If you INSTEAD want to consider the displacement of the guy’s CENTER OF MASS then you’ve got PSEUDOwork and we’re into mechanics not thermodynamics. (It’s just Newton’s second law in disguise and no thermo is involved.)

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Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9b, 572C Holloway Rd, Annapolis MD 21402-1363 
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/