1M40.99 - Onager

See procedure
Code Number:
Demo Title:
Energy Conservation
Area of Study:
Onager, Ping Pong balls.

Wind up the strings on the onager being careful that you wind both sides equally and in the right direction.  These will have to be wound very tightly for this to work well.  Place a Ping Pong ball into the spoon, pull the arm back and set the trigger.  Release the trigger and the ball should travel about 20 feet. 

  • Mark Denny, "Optimum Onager: The Classical Mechanics of a Classical Siege Engine", TPT, Vol. 47, # 9, December 2009, p. 574.
  • Kelvin Catapult Design P/N 841860, Kelvin School Projects.
  • Philip Friedman, "Tool Test", Pop Mechanics, Aug. 2012, p. 97.

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.