 ## Sunday, 30 January 2011

### Super Balls

Which balls will bounce highest?

Raise the stacks of balls up and release them so that the balls will fall as straight and freely as possible, without shaking the cables. Do not throw the balls upwards.

In the stack in which the largest ball is the uppermost ball, the balls will hardly bounce at all. In the other stack, the uppermost small ball in the other stack bounces metres in height.

As the stack of balls collides with the base, the biggest ball at the bottom of the stack bounces upwards. A split second later, it collides with the next ball. At the point of collision, the momentum of the larger ball transfers to the smaller ball to such an extent that the bounce speed of the smaller ball is nearly tripled. When the same is repeated two more times, the speed of the smallest ball could theoretically be up to 27 times (=3x3x3) greater than the falling speed of the balls.
The height of the upward bounce is proportional to the speed when squared. If the balls were dropped from a height of one metre, the smallest ball would bounce to a height of 27^2 = 729 metres – in theory that is, when friction, air resistance and the balls’ only partial elasticity are left out of the equation. During the collisions in the second stack, the momentum of the uppermost larger ball is downward. For this reason, the balls aim downward as a result of the collision.

When a large and small object collide elastically with one another, the smaller object gets a larger speed than the combined speed of both objects. The hardest trajectory in football is achieved when a player kicks at a ball coming towards him.

Links

Super Balls in slow motion
A short history and physics of Super Ball
Another short history and properties of Super Balls