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Pendulum of Doom

When we lift a pendulum, we transfer energy to it. This energy allows it to swing back and forth when we let it go. If we don't add any more energy, the ball will swing less and less until it finally stops. The arc of the pendulum will never be longer than the first arc when we let the ball go. It is this "conservation of energy" which means that the pendulum of doom will come close, but never actually hit you!

This activity is part of Science World's Big Science for Little Hands program. Activities were developed and tested with Preschool and Kindergarten educators.


  • Use ramps, pulleys, levers and pendulums.


  • Per Student:
    Large soft ball with handle (e.g. a dog toy from a pet store such as Softflex Best Ball Gripper)
    Length of rope (enough to reach from the ceiling to the floor)
    Masking tape

Key Questions

  • What will happen when we let go of the ball?
  • What will happen if we leave the ball swinging?
  • Do you think the ball will come back and hit me?
  • Why won't the ball swing any further?

What To Do

  1. Tie one end of the rope to the ball.
  2. Tie the other end of the rope to a secure point in the ceiling (e.g. a light fixture or hook) so that the ball hangs about 60cm off the floor. The ball must be able to swing freely.
  3. Demonstrate that this is a pendulum, just like the ones they have already experimented with, by swinging the ball back and forth.
  4. Make a line on the floor where you are standing using the masking tape.
  5. Keeping the rope taut, pull the ball up to just in front of your face. Let go and don’t move!
  6. Repeat step 5, inviting a child to stand behind the line. Bring the ball up to their face and let go, making sure they don’t move.
  7. Allow others to have a turn, one at a time.


  • In the playground, investigate different kinds of swings – they are pendulums, too!

Other Resources

Science World | YouTube | Exploring the Conservation of Energy (Pendulum Fun!)