Jessie Maher, of Balclutha Primary School asks :-

Why do some balls bounce?

Blair Blakie, a physicist at the University of Otago, responded.

Let us consider what happens when we drop a ball onto the floor. The ball accelerates under the influence of gravity, and as it speeds up it accumulates energy associated with its motion, known as kinetic energy.

How well the ball will bounce depends on what happens to this energy during the collision with the floor. In the collision kinetic energy is transformed into deforming the floor and the ball (i.e. squishing it), and the ball is instantaneously motionless. Because the period of time that the ball is in contact with the floor is so short, typically a few thousandths of a second, we are hardly aware of this deformation when we observe a bouncing ball. If the ball and floor are elastic they will tend to restore themselves to their original shape. In doing this, the energy stored in the deformation (much like the energy stored in a compressed spring), is used to put the ball back into motion, that is, it bounces. This is the case for tennis balls and basketballs bouncing on hard floors.

However, if the ball (or the floor) cannot efficiently return the energy after being deformed, then the ball will not bounce very well. For example, consider a ball made from soft putty; When it hits the ground the putty will deform, but has no tendency to restore itself to its original shape, and most of the energy is converted to heat. This is also the case for most sports balls when they go "flat".