How to Build a Pendulum

Pendulums are used in clocks, music timing devices, experiments and even art! The Foucault pendulum is the most famous type. Named after its inventor, Leon Foucault, it would swing all day, but instead of swinging in the same direction, it seemed to rotate slowly in a circle. This proved that the Earth rotates [source: Science Daily].

Although the original Foucault pendulum was very large, you can make a smaller version. However, you'll have to do this outdoors on a windless day. Here's what you'll need:


  • Cloth or canvas sack
  • Fine, dry sand
  • Large garbage bag
  • Nail
  • Nylon cord, 13 feet (4 meters) long
  • Tall stable structure, such as a 12-to-15 foot (3.7-to-4.6 meter) ladder or swing set
  • Tape

Here's how to make a Foucault pendulum:

  1. Fill the sack with sand.
  2. Check that the sack has no holes by lifting it up and gently shaking it.
  3. Seal the sack with the cord.
  4. Hang the sack from the high structure.
  5. Poke a hole in the bottom of the sack with the nail. Make the hole a little larger than the tip of a pen, so the sand doesn't come out too fast. Check the rate of flow -- you should see a slow, steady stream. Adjust the size of the hole if necessary.
  6. Seal the hole with tape.
  7. Spread a garbage bag under the pendulum, so it covers the surrounding area.
  8. Check that the sack is hanging straight down. Adjust the cord or sack if necessary.
  9. Pull the sack back while keeping the cord taut until it's around 4 feet (1.2 meters) above the ground.
  10. Remove the tape from the hole and carefully start the pendulum swinging in a straight line. It shouldn't swing in an ellipse.
  11. Let the sack swing for an hour, gently pushing the cord whenever it starts slowing down. Be careful to push it in the direction of the swing; don't shift or redirect the cord.
  12. Watch the sand traces on the garbage bag -- each trace should be slightly offset from the previous one. The direction and distance between each trace depends on the hemisphere and latitude of your location [source: Foucault Pendulum].

Be patient; it make take more than one try to get it right.