So, here’s the obvious next question: Is kinetic energy also conserved, just like momentum is conserved? The answer is: sometimes. For some collisions that we call “elastic collisions,” both kinetic energy and momentum are conserved. In general, elastic collisions happen between very bouncy objects—like two rubber balls, or pool balls colliding. If we have an elastic collision in one dimension (meaning everything takes place in a straight line), then we have two equations that we can use: conservation of momentum and conservation of kinetic energy.
In addition to elastic, there are two other kinds of collisions. When two objects collide and stick together, like a lump of clay hitting a
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