165 – Bouncy Ball

Elastic bouncing ball

Introduction to:

Energy transformations, potential energy, kinetic energy, strain energy, elastic impact

Material:

Bouncy ball / tennis ball / elastic ball
yardstick / meter stick
Cell phone with video function / video camera

Execution:

We drop the ball in front of the yardstick from a height of about one meter onto a hard surface and let it bounce up several times. While doing this, we film the process in slow motion.

Observation:

The ball accelerates during free fall.
On impact, its velocity’s direction reverses.
It moves upwards, then it slows down again and comes to a stop at the top for a short time.
In the slow motion video, and even better in a still image, the altitude of the ball can be read from the yardstick.
Now the free fall begins again.
The altitude decreases after each bounce on the ground.

Explanations:

The gravitational force acts downwards.
On impact, the ball is deformed elastically and experiences an upward push.
Now only the gravitational force acts until the next bounce.
From an energy standpoint, the potential energy of the ball is being transformed into kinetic energy bit by bit with each bounce.
On impact, the kinetic energy is converted into strain energy, which is then converted back into kinetic energy, which is transformed into potential energy, back into kinetic energy and so on.
From the decreasing altitude, we can infer a loss of mechanical energy that occurs at each bounce.
When the ball is deformed, friction is created and thus thermal energy dissipates.

Didactic potential:

It is interesting to ask whether the same percentage of mechanical energy is lost at each bounce. This can be seen in the respective altitudes.