This afternoon I asked my 6 year old twin sons if they would like to investigate how fast objects fall. They liked the idea, but they liked it even better when I told them they would be allowed on top of the roof to drop the falling objects. We had two questions:

  1. What falls faster: a heavy or a light ball?
  2. What falls faster: a big or a small ball?
In order to answer the first question we cut an old tennis ball open, filled it with folded lead plates and glued it shut again. For the second question we took a small basketball that was about the same weight as the lead-filled tennis ball.

Left: Normal tennis ball of 47 grams. Middle: Lead filled tennis ball of 245 grams. Right: Basketball of 235 grams.

After weighing, the twins started dropping the balls in pairs from the roof, while I made a movie with my phone. The VideoPix app was used to grab all movie frames at a framerate at 30 frames per second. Below you can see the results after cropping the frames and putting them in a single figure.

Top: Comparing the falling speed of a lead-filled ball left and a normal tennis ball on the right side of the frames. Bottom: Comparison of a lead-filled tennisball and a small basketball. The roof height is measured below to be 2.85 m, the time between consecutive frames is 1/30 of a second. The white lines are parabolas z=z0-1/2 g t^2 with g=9.81 m/s^2.

We took several movies. Sometimes the boys did not release the balls at the same time. The movies shown in the figure above were the ones with the smallest difference in arrival time. Within the measurement accuracy we thus conclude that:

  1. Heavy and light balls fall at the same speed.
  2. Big and small balls fall at the same speed.

The lines in the figure above show that the balls reasonably seem to follow the theoretical curve:

z=z_0-\frac{1}{2} g t^2

Measuring the height of the roof to be 2.85 m.

Science with kids

Since I now have this site for more than a year, ask I thought I should generate some interesting content for it. Therefore I have decided to perform some simple scientific experiments together with my sons Jeroen and Sander and document them on this site. Hopefully you will find it fun to read and are motivated to repeat the experiments.

For my professional scientific work, malady please visit:

Peter G. Steeneken