Elastic and inelastic collisions
As in all collisions, momentum is conserved in this example. But calculations comparing kinetic energy before and after the collision show kinetic energy is not conserved. This is an inelastic collision.
If kinetic energy before is the same as after, then the collision is elastic. Interactions between molecules are examples of perfectly elastic collisions. In most other cases (eg snooker balls), collisions are not perfectly elastic - some kinetic energy is lost. But this must be tested by calculation.
Remember that kinetic energy can be calculated using the following equation:
\({E_k} = \frac{1}{2}m{v^2}\)
When working with collisions, kinetic energy must be worked out for each object involved both before and after the collision.
Question
If two bumper cars collide head-on in a fairground and both cars come to a stop due to the collision, kinetic energy is obviously not conserved. Is momentum conserved even though both cars stop?
Yes, although there is no momentum after the collision, there was no total momentum before the collision. Momentum is a vector quantity. The positive momentum of one car must have been balanced out by the negative momentum of the other car.
Question
Is kinetic energy conserved even though both cars stop?
No. Kinetic energy is a scalar quantity so there is no balancing out of the different directions. Both cars were moving and had kinetic energy before the collision. Both cars are at rest and have no kinetic energy after.