That's a great question for two reasons:
1) It involves interesting physics.
2) You can do experiments by yourself to verify the stuff that I will write below (please try it).
The best way to think about bouncing balls is to first think about something simpler that is also bouncy: A SPRING.
you have a spring that looks like |/\/\/\/\/\/\/\/\/\/\|. If you grab
both ends and push them together, the spring compresses. When you let
go the spring will push itself back out to its original length. The
physics that goes on is the following: When you compress the spring you
are putting energy into it, energy that it stores. When you let go of
the spring, it uses that energy to get back to its original length. The
energy stored in the spring is called "Potential Energy" since it has
the potential to do stuff (like push your hands apart). After you let
go of the spring, and it has expanded back to its original length, all
of the potential energy you put into it has been released….the spring
is exactly the same as it was before you squeezed.
A ball is
a lot like a spring. When you throw a tennis-ball against a wall, it
gets squished (in the same way that the spring can be compressed). If
you had a high-speed camera and took a picture at the instant the ball
was at rest, just before starting to bounce back, you would see that it
was quite deformed. This deformation stores potential energy in the
rubber the ball is made of, which is then released back to the ball
causing it to bounce back as it becomes round again, having almost the
same speed you threw it with.
The key word is ALMOST. The
material the ball is made of (rubber or plastic perhaps) is not an
ideal spring like the case we discussed above. When we put energy into
a ball by deforming it we don't get all of it back again when it
returns to its original round shape…this is just a property of most
materials. (Springs are also not truly ideal, but they are usually much
better at releasing their potential energy than balls are).
the more something deforms as it bounces, the worse it is at giving
back its energy…a hard ball generally bounces higher than a soft
one…you know this is certainly true for soccer balls you have played
with. Super-Balls (the ones that bounce really high) are made of very
hard solid rubber.
OK, now suppose you throw a ball against a
wall of a floor that is itself squishy rather than hard (like a
carpeted floor versus a hardwood floor). Now the floor also acts like a
spring, and just like the case of a ball, a squishy floor is probably
not so good at giving back its deformation energy. If you bounce a
soccer-ball off a hard road it will bounce higher than if you bounce it
off a lawn.
By the way, just because the potential energy is
not all released into the motion of the bouncing ball (called kinetic
energy), it is nevertheless not lost…energy can not be created or
destroyed, just changed. The potential energy that is apparently not
recovered as motion of the ball becomes heat (the ball warms up a bit
as you bounce it) and perhaps also sound (the ball makes a loud noise
as you bounce it).
(republished on 07/11/06)