Elastic and Inelastic Collision Model

What it shows:

Two cars have the same mass and same spring bumper. When given a push and allowed to collide with a wall, one car bounces off with only a small reduction in speed ("elastic" collison) whereas the other car comes nearly to a complere stop ("inelastic" collision).

How it works:

There are two impulse cars made of identical materials and have the same mass. The car that models an elastic collision has all its lead sinkers securely attached to the frame so that they can't move. In contrast, the car that models an inelastic collision has the lead sinkers suspended from swivels so that they are able to jostle and swing around. Both cars have low-friction wheels and a small superball that serves as a very elastic bumper.

impulse cars impulse cars

When the "elastic" car hits a wall, it rebounds almost perfectly. On the other hand, when the "inelastic" car hits a wall, all the suspended lead sinkers are set in motion randomly jiggling and swinging about ... the car itself hardly rebounds at all. The initial kinetic energy of the car has been transformed into random kinetic energy of its contents (which models internal thermal energy).

Setting it up:

The lecture bench top is ideal for this demo. A large heavy bar of iron, C-clamped to the bench, serves as "the wall." Make sure the bench is level ... you don't want to be embarassed to discover the inelastic car rolling away from the wall.


You may wish to follow this up with a pair of "happy" and "unhappy" balls. The unhappy ball doesn't bounce ... ask students where the kinetic energy goes for the unhappy ball after the collision.