1C20.10 Coin and Feather (Thor's Hammer)


Acceleration of gravity, drag


A 6-feet long Plexiglas tube containing a "coin" and "feather" is evacuated. The "coin" and "feather" fall at the same rate. When air is introduced into the tube, the "feather" falls at a slower rate. Note: The vacuum tube is large, but the "coin" and "feather" are difficult to see from the back of a large lecture hall.



  • [1] Plexiglas tube containing a packing peanut and wrapped washer
  • [1] Vacuum pump
  • [1] Power bar
  • [1] Extension cord 

Classroom Assembly

The instructions assume the demo will show free-fall in air first, and then in vacuum second. The tube must also be pumped down before class if starting with free-fall in vacuum.

  1. Place the tube somewhere with lots of room around and above you.
  2. Open the valve on the tube.
  3. Plug in the pump.

Important Notes

  • This demonstration requires advanced practice. If the tube is not turned smoothly, the "feather" can grip the inside of the tube and fall more slowly in a vacuum.
  • For best results, hold the tube horizontally and shake it gently so that the "coin" and the "feather" are side-by-side on the wall of the tube. Quickly and smoothly rotate the tube to the vertical.
  • A few problems can occur. Very occasionally, the objects can become stuck at one end of the tube. Also very occasionally, the two objects interfere with each other while falling. The recommended gentle shake in the horizontal position makes these occurrences very unlikely.
  • Evacuating the tube takes a few minutes. Some people choose to evacuate the tube in advance and show the evacuated state before the air resistance state to eliminate pump-down time.


  1. Pick up the tube and hold it vertically so the "coin" and "feather" fall to the bottom.
  2. Repeat as necessary. Point out that the feather falls at a slower rate than the "coin."
  3. Attach the pump to the tube. Make sure the valve on the tube is open.
  4. Turn on the pump and let it pump until the pressure in the tube has reached a stable low value.
  5. Close the valve on the tube. Turn off and disconnect the pump.
  6. Pick up the tube and hold it vertically so the "coin" and the "feather" fall to the bottom.
  7. Repeat as necessary. Point out that the "coin" and "feather" fall at the same rate.
  8. [Optional] Let some air back into the tube and repeat the experiment to show the effect of reduced pressure. This will demonstrate that a good vacuum is essential for the success of this experiment

Additional Resources


  • PIRA 1C20.10


  • Don't attempt this at home!
  • SFU is not affiliated with any external sites linked here and is not responsible for their content.

Last revised

  • 2018


  • The tube was constructed in the SFU machine shop. It was made from a 6' x 4" plexiglass tube capped off and sealed on both ends by aluminum plugs with double O-rings. One end was fitted with a vacuum connection to allow evacuation of the tube. A large washer is used in place of a coin and a packing peanut is used in place of a feather. An Edwards single stage mechanical pump is used to evacuate the tube.

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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.