When you drop a ball, gravity pulls it toward the floor.
The ball gains energy of motion, known as kinetic energy. When the
ball hits the floor and stops, that energy has to go somewhere. The energy
goes into deforming the ball--from its original round shape to a squashed
shape. When the ball deforms, its molecules are stretched apart in some
places and squeezed together in others. As they are pushed about, the molecules
in the ball collide with and rub across each other.
Exactly what happens to these molecules as they stretch
and squeeze depends on what the ball is made of. Suppose you drop a ball
of putty. Rather than bouncing, it hits the floor and flattens. All of the
organized motion of the falling ball becomes the random motion of jiggling
molecules. The random motion of jiggling molecules is a measure of thermal
energy. The putty gets warmer, but it doesn't bounce. Putty is inelastic--it
doesn't return to its original shape.
Now suppose you drop a rubber ball. Rubber is made from
long-chain polymer molecules. When you hold the ball in your hand, these
long molecules are tangled together like a ball of molecular spaghetti.
During a collision, these molecules stretch--but only for a moment. Atomic
motions within the rubber molecules then return them toward their original,
tangled shape. Much of the energy of the ball's downward motion becomes
upward motion as the ball returns to its original shape and bounces into
the air. The energy in the ball that isn't converted into motion becomes
warmth. (You can verify this the next time you play a game of racquetball.
At the end of the game, the ball will be warmer than when you started.)
Rubber balls are elastic because they return to
their original shape. But rubber polymers can be formulated in different
ways: if the polymers are tightly linked, they do not rub against each other
much. The organized motion of the falling ball becomes an organized deformation
of the rubber of the ball, which then becomes an organized motion of the
bounced ball. Very little of the organized motion is lost by warming the
ball; most of it goes into bouncing the ball back into the air. Balls made
from this type of rubber are called "superballs." On the other
hand, rubber polymers can be made in which the molecules move more freely,
rub together more, and turn organized motion into disorganized vibration.
The ball will hardly bounce. Instead, it gets warm.