Kepler's Second Law
Earth and Space

Kepler's Second Law with a whirling bung

Practical Activity for 14-16 PRACTICAL PHYISCS

This simple class activity demonstrates the increase in speed with a diminishing radius of orbit.

Apparatus and Materials

For each student or group of students

  • Rubber bung
  • Length of twine

Health & Safety and Technical Notes

Read our standard health & safety guidance


  1. Attach a length of twine securely to the rubber bung. Whirl it round in a circle.
  2. 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

  • Kepler's Second Law states that a planet in its orbit sweeps out equal areas in equal times. The time of orbit of the puck will fall as its radius of orbit decreases.
  • You can measure the load by attaching a newton meter to the bottom of the string. As the force on the puck is increased, its orbit gets smaller and its speed increases.
  • 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.
  • 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.
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