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Unformatted text preview: involved, such as in a car crash. We define an inelastic collision as a
collision in which momentum is conserved, but kinetic energy is NOT. The collision of a rubber ball
with a hard surface is inelastic, because some of the kinetic energy is lost when the ball is deformed
during contact with the surface. When two objects collide and stick together, the collision is called
perfectly inelastic. For example, if two pieces of putty collide, they stick together and move with some
common velocity after the collision. If a meteorite collides head on with Earth, it becomes buried in the
Earth and the collision is considered perfectly inelastic. Only in very special circumstances is all the
initial kinetic energy lost in a perfectly inelastic collision.
An elastic collision is defined as one in which both momentum and kinetic energy are
conserved. Billiard ball collisions and the collisions of air molecules with the walls of a container at
ordinary temperatures are highly elastic. Macroscopic collisions such as those between billiard balls are
only approximately elastic, because some loss of kinetic energy takes place—for example, in the clicking
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