1F20.10 Inertia Ball


Acceleration on an object


A weight is hung from a frame by a piece of thread. An aluminum rod is attached to the bottom of the weight by a second piece of string. The aluminum rod is pulled down. If the rod is pulled down slowly, the top thread will break first. If it is pulled down quickly, the bottom thread will break first.



  • [1] 3 kg spherical weight with hooks on the top and bottom
  • [2] Thread
  • [1] Rigid frame
  • [1] Aluminum rod
  • [1] Heavy-duty string

Classroom Assembly

  1. Attach the weight to the frame using the heavy-duty string as a safety line.
  2. Hang the weight from the frame using a piece of thread. The thread should be shorter than the safety line so that the thread, not the safety line, is taught.
  3. Attach a second piece of thread to the bottom of the weight.
  4. Attach the aluminum rod to the bottom piece of thread.

Important Notes

  • Improper use of this demonstration can result in injury. Ensure that the safety line is firmly attached to the weight and the frame.


  1. Pull the aluminum rod down slowly. The top thread will break. Note that the weight will be prevented from hitting the table (or your feet!) by the safety line.
  2. Repeat steps 2-4 from the classroom assembly.
  3. Pull the aluminum rod down quickly. The bottom thread will break.


Additional Resources


  • PIRA 1F20.10
  • Video Encyclopedia 2-13
  • Sutton M-100; DHP Mc-2; DaR M-250


  • Don't attempt this at home!

Last revised

  • 2018


  • Original construction: all parts were assembled from surplus lab equipment.

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.