I'm uncomfortable with the "momentum transfer but no
energy transfer" model described in earlier posts.
Consider a similar situation, the hypothetical
perfectly elastic collision between a fall ing ball
and the floor. Model the floor as an ideal spring.
During the first half of the interaction, as the
"floor spring" is being compressed, the floor does
"negative work" on the ball, thereby reducing both its
KE and momentum to zero (in the floor ref frame).
During the second half of the interaction, as the
"floor spring" is recovering it is doing "positive
work" on the ball, increasing both its KE and momentum
to their original absolute values. I think it's pretty
common (and also pretty clear to intro students) to
treat the force of the floor in this fashion, no? We
don't really care where the energy came from that
enables the "floor spring" to do its thing (but it was
from the gravitational interaction of the ball and the
Earth).
I haven't thought the accelerating car (or
accelerating person) situation completely through, but
so far it seems that the analogy is strong. Your foot
(or tire) is able to push back on the floor (or road)
b/c of friction, thereby deforming the surface. As the
surface recovers, it does work on you (or the tire)
thereby increasing both the KE and momentum (again in
the road or floor reference frame). If we are content
to say the floor does work on the bouncing ball, I
don't see why it is any less correct to say the road
does work on the tire. For the car, the source of the
energy that permits the surface deformation is the
chemical reaction in the engine, but doesn't that
energy provide the means to deform the surface so it
can push back on and accelerate the vehicle? John Barrere
__________________________________________________
Do You Yahoo!?
Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com