Re: Loss of KE

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 22:43 -0500 11/26/02, David Abineri wrote:
> In an ideal inelastic collision (objects stick together) between two
> equal masses with one stationary initially, one half of the KE is lost
> (transferred to another form).  Why is it that this amount of lost KE
> does not depend on the materials of which the objects are made? It would
> seem that one should get different results if they were made of steel vs
> foam rubber for example.
>
> Is this simply because this is an ideal situation or is there more to it
> than this?
Because the nature of the materials determines whether the collision
can be inelastic or partially elastic. If you contrive the objects so
that they stick together by putting velcro on them or instant glue or
hooks or some other means then the loss for the situation you desribe
is 1/2 the initial KE. And that is because the velocity is half its
former value and the mass is twice its former value, so the increased
mass can only negate half the decrease velocity when you calculate
energy, but they do negate each other in momentum.

There is nothing more to it than this. The determination of the
nature of the collision depends on whether or not you can make them
stick together or not. That is a function of the materials unless you
do something else to make them stick or keep them from sticking.

But in the center of mass reference frame all inelastic collisions
always lose 100% of their KE, because in that ref. frame the motion
stops. How much KE is lost in an inelastic collision depends on the
relative masses of the two objects and the reference frame from which
it is viewed.

All collisions except perfectly elastic ones lose some KE. The amount
lost depends on the degree of elasticity and the frame from which you
view the collision.

Hugh
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Hugh Haskell
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