Re: Pb

[Jim Green <JMGreen@SISNA.COM>]

> But the ball stops bouncing eventually. Each bounce is lower than the
> previous. I
> am trying to show where (some) of that "missing" energy is going.

Leigh is not here any longer to chime in; may I substitute?

If we would stop saying and thinking things like "energy going" an start
realizing that the energy can _change_ but not _move_, these issues would
become clearer.

The ball has the property of "energy". This property can change in
magnitude ONLY if work is done on/by the ball.  AND the thing which does
that work also changes the magnitude of its energy.   Note N#3.

The Earth does work on the ball. (~+mgh)  The ball does work on the
Earth.  The Earth moves. The ball hits the floor.  A real floor does some
work on the ball.  (A hard floor does not)  The ball does some work on the
real floor.  A real ball also does some work on itself (cf the teachings of
John Denker and the lesson of the ice skater hitting a rigid wall)  (A hard
ball dos not) The ball bounces and repeats the process.

During this action work is done on the floor and the ball.  Some of this
work increases the internal energy of the ball and some of this work
increases the internal energy of the floor.

Eventually the total increase in the internal energy of the floor and of
the ball ~=mgh

The answer to all of the so called "energy flow" questions is buried in the
work/energy law/theorem.

None of the "energy" is "lost" but it doesn't "go" anywhere either.  Work
is done on various parts of this complicated system by the various parts of
this system and the energy of these parts is increased or decreased, but
the energy does not move.  Energy is a property of a system or its
parts.  One would not likely say that blue moves.  Blue paint can move, the
ball can move, but energy does not move by itself.

Now I don't claim that Leigh would say it just this way, but I do think
that he would agree -- at least in the past year or two.


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen