Kepler's Second Law with a whirling bung
Apparatus and materials
For each student or group of students
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.
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.
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.