What makes things bounce?



Besides the material of the object, what causes things to bounce? For example, a bowling ball and a feather won’t bounce (assumption) on a concrete floor. Neither will a flat piece of paper but once you crumple it, there’s a chance it will bounce. Talking specifically about free falling objects, not thrown or anything with force applied to it. Does the distance of the drop also affect if something will bounce?

In: Physics

For every action there is an equal and opposite reaction. When an object hits the ground after falling, the ground imparts force back into the object.

The material of the object is very important in determining whether it will bounce, and how high. A rubber ball, for example, is made of material that will deform when it impacts the ground, then quickly return to its original shape (thus pushing it against the ground). It works in a similar way to a spring – it gets compressed, building up potential energy, then pops back to its original shape, releasing that energy into the ground and pushing itself up, thus ‘bouncing’.

When you drop something it starts off at 0 speed, but with lots of potential energy. While falling it turns that potential energy into kinetic energy.

When it hits the floor it still has that kinetic energy and it has to go somewhere. Some of that turns into heat, and some into sound. And a lot of the energy is used to deform the floor and the falling object a little bit.

This deformation depends on the materials involved. A pingpong ball will squash easily, and then when it un-squashes itself, it pushes against the floor and bounces back up.

The only thing the distance changes (in most cases) is how high it bounces. The bounce itself all depends on the materials involved.

A moving object carries energy in its motion. Upon impact, that energy *must* be absorbed.

A sandbag being dropped on concrete will absorb its energy mostly in the friction of the sand grains. This converts its energy to heat – it will not convert from heat back into motion.

A rubber or play-doh ball hitting a tile floor will absorb its energy in the flattening of the ball. However, the rubber ball will absorb it as an elastic stretching energy, like a spring, while the play-doh ball will absorb the energy by *inelastic* stretching – this produces heat. While the rubber ball returns to its original shape, the play-doh ball remains flat. This is a sign that rubber is far more elastic than play-doh.

So, the rubber ball absorbs the energy as an elastic stretching, and uses the energy to return to its original shape, which also involves firing the ball back up into the air as it snaps back into its original shape.

In any collision, the energy absorbed through elastic versus inelastic deformation depends quite a bit. A steel ball on a steel surface at low speeds will behave extremely elastically, but at high speeds it behaves almost completely inelastic.