Just a short comment.The derivation from Leigh Palmer is correct only
for point particles. Whenever the body is not strictly rigid (neither a
a table nor the brick) the derivation has to be made for the very small
parts of the brick and the table separately. External forces for each part
are now also internal forces inside of the brick (and or the table). Most
of them are of course unknown and are phenomenologicaly taken as friction
and th cause of the chaotic movements of the parts as heat. The correct
analysis of the mater is beyond the computer capacities etc..
So, it is much more valuable to treat brick separately from the table with
a work going in and out of it when using only mechanical energy and the
simplification of the brick as a point particle.
To treat also the change of internal energy we have to take as a system
the brick and the table. Now, there is only one external force doing work
and consequently the internal energy changes have to be assigned to the
brick AND to the table. And there is NO heat transfer (if we forget the
higher temperature of the brick table sistem than the surroundings).
Mojca Cepic
Faculty of Education,
Ljubljana,
Slovenia