3A95.70 - Variable Length Pendulum - Swing - Pump a Swing

Code Number:
Demo Title:
Variable Length Pendulum - Swing - Pump a Swing
Potential to Kinetic Energy, Simple Harmonic Motion
Area of Study:
Variable length pendulum, variable length pendulum with aluminum bar and bearing supports.

The variable length pendulum makes use of a plastic bushing at the pivot point to make the pendulum length variable while the pendulum is swinging. This is often used to demonstrate subtle points.

The variable length pendulum with bearing supports is a more stable system designed to withstand the forces of the swinging pendulum.  This needs to be set up on one of the little tables that has the rod supports and the proper distance.  Even then if the pendulum is given enough energy the whole table will visibly move when the length of the pendulum is varied.

  • Ann-Marie Pendrill, "Serious Physics on a Playground Swing - With Toys, Your Own Body, and a Smartphone", TPT, Vol. 61, #5, May 2023, p. 355.
  • Klaus Bickel and Robert Denschlag, "Playground Physics: What Happens When Trying a 360° Swing?", TPT, Vol. 57, #3, Mar. 2019, p. 146. 
  • T. Wickramasinghe, and R. Ochoa, "Analysis of the Linearity of Half Periods of the Lorentz Pendulum", AJP, Vol. 73, #5, May 2005, p. 442.
  • William B. Case, "The Pumping of a Swing From the Standing Position", AJP, Vol. 64, #3, Mar. 1996, p. 215.
  • William B. Case and Mark A. Swanson, "The Pumping of a Swing from the Seated Position", AJP, Vol. 58, #5, May 1990, p. 463.
  • Stephen M. Curry, "How Children Swing", AJP, Vol. 44, #10, Oct. 1976, p. 924.
  • John T. McMullan, "On Initiating Motion in a Swing", AJP, Vol. 40, #5, May 1972, p. 764.
  • Bryan F. Gore, "Starting a Swing from Rest, "AJP, Vol. 39, #3, March 1971, p. 347.
  • Joseph A. Burns, "More on Pumping a Swing", AJP, Vol. 38, #7, July 1970, p. 920.
  • Bryan F. Gore, "The Child's Swing", AJP, Vol. 38, #3, Mar. 1970, p. 378.
  • A. E. Siegman, "Comments on Pumping on a Swing", AJP, Vol. 37, #8, Aug. 1969, p. 843.
  • Peter L. Tea, Jr. and Harold Falk, "Pumping on a Swing", AJP, Vol. 36, #12, Dec. 1968, p. 1165.
  • Jearl Walker, "1.115, The Pendulum in the Pit", The Flying Circus of Physics Ed. 2, p. 57.
  • Jearl Walker, "1.113, Pumping Playground Swings", The Flying Circus of Physics", p. 56.
  • Jearl Walker, "1.114, Incense Swing", The Flying Circus of Physics", p. 56.
  • Ron Hipschman, "Pendulum Table", Exploratorium Cookbook III, p. 188.1 - 188.18.

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