Two Types of Collisions
When bumper cars collide, they bounce off each other. Most of the
force goes into changing the motion of the cars. The two bumper cars
travel separately after the collision, just as they did before the collision.
The combined momentum of both cars after the collision is the same
as the combined momentum of both cars before the collision.
In this crash test,
momentum is conserved,
but some of the energy
goes into bending the
metal in these two cars.
When two cars collide during a crash test, momentum is also
conserved during the collision. Unlike the bumper cars, however,
which separate, the two cars shown in the photograph above stick
and move together after the collision. Even in this case, the total
momentum of both cars together is the same as the total momentum
of both cars before the collision. Before the crash shown in the photograph, the yellow car had a certain momentum, and the blue car had
no momentum. After the crash, the two cars move together with a
combined momentum equal to the momentum the yellow car had
before the collision.
Compare collisions in which objects separate with collisions in
which objects stick together.
Momentum and Newton’s Third Law
Collisions are not the only events in which momentum is conserved.
In fact, momentum is conserved whenever the only forces acting on
objects are action/reaction force pairs. Conservation of momentum is
really just another way of looking at Newton’s third law.
388 Unit 3: Motion and Forces