# Galileo’s Cannon and Supernova

Originally shared by Larry Phillips

Galileo’s Cannon and Supernova

This animation shows a counter-intuitive phenomenon, but one which can be explained using energy and momentum conservation.

Two balls fall together, the small one being much less massive than the larger one. The large one hits the ground and rebounds. It immediately collides with the smaller one, which is still moving downward. The collision sends the smaller ball upward to a height nine times the height from which the balls were dropped. Energy is still conserved: the small ball’s increase in energy comes from a reduction in the large ball’s energy.

This is sometimes called “Galileo’s Cannon”. I recently used two rubber balls to demonstrate the effect, and it’s a striking thing to see. I drilled a small hole through each ball. A thin rod passes through the holes, and this keeps the balls aligned vertically as they fall.

Two examples that involve similar physics:

Type II Supernova explosion. When the star runs out of fuel, its core and surrounding gas begins to fall inward. When the core reaches maximum density, it rebounds, then collides with the in-falling gas. The gas is then blown outward at tremendous speed.

The gravity slingshot maneuver. This is effectively a collision between a spacecraft (the small mass), and a planet (the large mass), though the only contact between the two is via gravity. The spacecraft “steals” some of the planet’s energy, and leaves at a much higher speed that it had before the encounter.

The second image shows a non-mathematical explanation of how this works.

#physics