Re: Car acceleration

[Bob Sciamanda <trebor@VELOCITY.NET>]

> When I first read this, I thought I got the answer (which was the
'correct'
> answer).  That is below.  Then I thought about it some more from an energy
> standpoint - now I'm not so sure.  What does the group think?

Consider a ping pong ball (mass=m, initial speed =v) bouncing elastically
off the (initially stationary) earth (mass=M).  In the limit as M/m
=>"infinity", the recoil velocity of the earth =>0, the recoil momentum of
the earth MV=>2mv, and the recoil kinetic energy of the earth .5MV^2=>0.
There is a momentum transfer, but no energy transfer.
In much the same way, the idealized cases of the skater pushing off the wall
and the turning car wheels "pushing off" the road surface involve a momentum
exchange but no energy exchange.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor