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Re: car acceleration



From: "John Barrer" <forcejb@YAHOO.COM>
I'm uncomfortable with the "momentum transfer but no
energy transfer" model described in earlier posts.

At the risk of repeating myself, this might help:
Consider a body with mass M and speed V. Suppose that in an interaction
with another object the speed V gets smaller and smaller as the mass M is
chosen larger and larger.

Then in the limit as M => infinity and V =>0 it can happen that the product
MV stays finite, non-zero and well defined. But then the limit of .5MV*V
will go to zero.

This is exactly what happens when a ping pong ball bounces elastically off
the earth.

In the limit of an infinitely massive earth, the earth acquires a definite
momentum equal to twice the original ping pong ball momentum, but the earth
acquires no KE because the product of the finite MV multiplied by another
vanishing V vanishes. There is an exchange of momentum, but no exchange of
energy.

It is true that in such an elastic collision, energy must be temporarily
stored, and then returned, in some mechanism (an intermediate spring, a
deformation of either/both objects, etc) but the overall reaction results in
no final energy exchange between the two objects and a well defined, finite
momentum exchange.

Speaking loosely, the force of interaction on the infinite mass results in
zero recoil velocity, finite momentum change MV, and zero KE change .5 MV^2.
This is required by the conservation of momentum. Blame Newton's laws.

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