In talking about work & energy with interested parties, one person
asked the following thoughtful question:
How come a full work-energy analysis of kinetic friction requires us
to consider the asperities ("microscopic doodads") on the bottom of a
sliding block, but no one ever talks about such doodads in a
treatment of say tension? Both involve molecular electrical forces,
so why is the literature asymmetric in their treatment?
Other examples of problems where you have to consider "doodads":
- figure skater pushing off a wall
- lump of clay hitting the floor
- compression of an ideal gas
In contrast, other examples of problems where the forces don't
involve "doodads":
- box in an accelerating elevator
- apple in freefall
- ping-pong ball fired out of a spring gun
The problems in the first list are those for which the pseudowork on
the object is not equal to the thermodynamic work; in the second
list, they are equal. Notice there are only nonconservative forces in
the first list. But there are also some nonconservative forces in the
second list, so that's not the key. (Even worse, the normal force is
in both lists and I suspect all nonconservative forces can be.)
Clearly the difference between the two lists is that the internal
energy of the object is being changed in the first list but not in
the second. So the question really boils down to:
How is that certain everyday forces change the microscopic energies
internal to an object, while other forces do not?
This question is either very profound or totally out of line. Either
way it's baffling me.
Again, I can tie a string to a moving block and manage to stop it
without hardly warming the block at all. But if I put sandpaper under
the block to stop it, I end up warming it to the point of scorching.
What makes tension and friction so different, when they're both just
electrical forces of attraction between adjacent molecules?
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/