Re: Loss of KE

[John Clement <clement@HAL-PC.ORG>]

> 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?
>
> Thanks for any insights,     David Abineri
>
>
>
> --
> David Abineri
> dabineri@choice.net

So far I have not seen any explanations which take into account the sticking
material.  The formalism certainly predicts the result for KE, but that is
extremely unsatisfying to many students.  They will tend to generate some
unfortunate misconceptions.

I think you need to look at what happens during the collision to see the
difference between soft and hard inelastic collisions.  If you have a soft
material such as clay it squishes gradually with a low force, over a long
period of time.  If you have hard material such as quick acting glue, the
squish is with a high force, very quickly.  In either case you do the same
amount of work on the material, and have the same loss of KE.  If you have
perfectly elastic materials, it is impossible to do work on the material,
and you end up with an elastic collision.  This reasoning also shows that
the KE went into internal energy in the material which produces the
sticking.  This is an extremely messy or even sticky problem which physicist
would like to ignore.

The formalism that predicts the loss of KE is valuable because you can
ignore the details of the collision, and just know whether the two objects
stick together.  Physicists love to find conservation principles that ignore
the details of a collision because it makes problems easier.  However in the
process visualization of the microscopic details may be lost.  This is
unfortunate because it leads students to be equation hunters rather than
problem solvers.

John M. Clement
Houston, TX

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.