1M40.15 Stopped Pendulum


Conservation of mechanical energy


A pendulum is released and the height of the swing is recorded. A rod is then placed in the way of the pendulum and the process is repeated. The height of the swing with the rod is the same as without the rod.



  • [1] 1.5" diameter wooden ball on a string
  • [1] Magnetic mount
  • [1] Rod on a magnetic mount
  • [1] Chalk
  • [1] Metre stick
  • [1] Magnetic blackboard or whiteboard

Important Notes

  • The pendulum may be made to loop the rod if the height of release and the height of the rod are chosen appropriately
  • Practice this demo in advance to drop the ball smoothly and avoid hitting the blackboard
  • Do not use a heavy bob as it may pull free from the mount or the blackboard


  1. Mount the pendulum on the blackboard.
  2. Raise the pendulum and mark the initial height on the blackboard.
  3. Release the pendulum.
  4. Mark the height of the swing on the blackboard.
  5. Place the rod below the pendulum mount so that it is in the path of the pendulum.
  6. Raise the pendulum to the marked initial height and release. The pendulum will swing to the same height as before.


Additional Resources


  • PIRA 1M40.15
  • VideoEnc 03-13
  • Sutton M-132;DHP Mr-3; DaR M-414; JSMiller p102


  • Don't attempt this at home!

Last revised

  • 2018


  • An eye hook was screwed into the wooden ball and a string was attached
  • The rod is a 4" length of aluminum bar

Related AV

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.