Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Kepler's Second Law with a whirling bung

Class practical


Apparatus and materials

For each student or group of students

Rubber bung

Length of twine

Health & Safety and Technical notes

Ensure that the bungs are securely attached and that each student has sufficient space to whirl the bung safely.


a Attach a length of twine securely to the rubber bung. Whirl it round in a circle.

using a whirling bung to demonstrate Kepler's second law
b After a few revolutions, allow the twine to wind itself up around the finger. As the length shortens, so the 'planet' moves faster and faster.


Teaching notes

1 This demonstrates Kepler’s Second Law. The arm from the Sun to planet sweeps out equal areas in equal periods of time. More distant planets move more slowly in their orbit. If you mark the position of a planet once a month on its elliptical orbit, and draw radii from the Sun to those points, the areas of sectors between those radii are all equal.
2 Kepler replaced constant speed of planetary motion (as with Ptolemy and the Greeks) with constant sweeping out of an area. Developing scientific explanations often involves a search for constant values.