1) All objects fall due to gravity. There is gravity between every
particle of matter. The equation for gravity is G*M*m/R^2 where G is
the universal gravitation constant, M & m are the masses of the
objects , and R is the distance between the centers of the objects.
Even though G is very small (G=.000000000067 in MKS units), and the
radius of the earth is pretty big (about 6.4 million meters), the mass
if the earth is so big (M=5980000000000000000000000 kg) that when you
multiply it all out the force of gravity on a ball is enough to make it
accelerate toward the earth quite rapidly.
2) The reason they stop bouncing is because they are loosing
energy. When you lift them up, you are giving them potential energy. As
they fall, the potential energy changes into kinetic energy. While the
object falls and as it moves back up after a bounce, the air is trying
to slow it down. That's like trying to walk through the water in a pool
or beach. It slows us down. Also, when the object bounces, it's
temperature goes up. You hear a sound as it bounces. And the object
vibrates a bit. All these things take energy away from the object.
Eventually, it will not have enough energy to go back up into the air.
3) The main reason for the differences you see are that the objects
are made of different materials. For example, a rubber ball bounces
better than a plastic ball. If you bounce a ball on hard surface, like
the floor, it will bounce better than on a bed. This is because the
springs in a bed help absorb the energy from the ball by moving while
they are touching. Also, some things don't move as well through the
air. A feather falls very slowly because the air slows it down quickly.
I hope this helps you out.
(republished on 07/11/06)