Completely Inelastic Collision on Air-track

We can experience this so-called totally inelastic collision on the air-track. We have carts coated with tacky wax which stick when they meet and hence the carts continue in motion together after the collision. We choose to let one cart be at rest initially. We measure the initial velocity of the one cart that does move, , and can thus predict the final velocity of the double cart to be

We compare to the measured velocity. We also calculate the amount of mechanical energy converted to internal energy:
 
We put in numbers for our particular experiment. From Eq. 11 we learn that the fraction of initial kinetic energy that can be converted to internal energy in a collision with an object at rest is

Two extreme examples do make sense:

1. If a car drives into a freight train at rest it will be bashed up and all motion will essentially cease: I have converted all the initial kinetic energy to internal energy in the wreckage.
2. If a freight train drives into a parked car the freight train barely changes its velocity and hence only a very small fraction of the kinetic energy is converted to internal energy in such a collision .