Experiment: Inelastic Collision

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Inelastic Collision
Experiment Inelastic Collision
Category Mechanics
Used sensors Microphone

Inelastic collision with a small steel ball.

The experiment "Inelastic Collision" listens for the sounds of a bouncing ball (this part is basically the same as the Experiment: Acoustic Stopwatch). The timing of the bounces is used to calculate the initial height of the ball, the height between each bounce and the portion of energy lost on each bounce.


The microphone is used to measure any sound. However, depending on your requirements you might want to attach an external microphone to your device to capture the trigger at its source.

Also note, that the watch will count too slowly if your phone is not fast enough to analyze the recording as fast as it is recorded. You can easily check this by comparing the acoustic stopwatch experiment to a regular clock. Do not worry: The watch will not go too fast on a fast device. If the device has a sufficient performance, the measurement will be accurate.


There is no specific setup. Pick a ball and a surface, that produces a well audible noise a few times.


The analysis is not too complicated, but uses a few if-modules to decide how many bounces have been heard so far. Otherwise, chunks of audio are analyzed for their maxima and compared to the threshold. If the threshold is exceeded, we calculate the sample number of the maximum and transform it into a time.

If the analysis takes longer than filling the audio buffer, samples are missed, so the watch won't work reliably in this case.

This so far is identical to the acoustic stopwatch experiment. From the timing the rest is calculated as follows:

The ratio of kinetic energy before and after a bounce equals the ratio of the squares of the time from this to the next bounce to the time the past to this bounce. The maximum height between two bounces equals 1/8 * 9.81 m/s² dt² with dt being the time between both bounces.

To get the inital height, the experiment assumes that the energy loss on the first bounce is approximately the same as on the second bounce.

Problems and resolutions

  • The measured time is too small. Unfortunately, this may happen if your phone cannot analyze the data as fast as it is recorded. At the moment, there is no solution to this except for using a faster phone.
  • Each bounce registers multiple times. In this case the duration of your noise is too long. The experiment waits at least 100ms before accepting a second trigger, but if there is an echo or a ringing to the noise, this can be sufficient to stop the clock again. You can try increasing the minimum delay.
  • Bounces are missed. In this case you probably need to decrease the threshold, so all your noises are above the threshold. You can use the audio oscilloscope experiment to check amplitudes.
  • The experiment triggers without actual bounces. Increase the threshold above the background noise level. You can use the audio oscilloscope experiment to check amplitudes.