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

Concepts

Acceleration of gravity, drag

Overview

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.

The "coin" and "feather" objects in the tube are difficult to see from the back of a large lecture hall.

Details

Equipment

  • [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.

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.

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.

Script

  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

References

  • PIRA 1C20.10
  • Video Encyclopedia 01-14
  • Mechanical Universe Episode 1 Chapter 15, Frames 13994 to 14476: Astronaut David Scott on the moon dropping a feather and a hammer, narration by Scott. The picture quality of this clip is very poor.
  • Mechanical Universe Episode 8 Chapter 18, Frames 17457 to 17769: very quick presentation of laboratory version, not worth showing separately. Part of Chapter 27, Frames 25196 to 25487: same footage as Episode 1, shortened, comments by the narrator.
  • The experiment was performed on the Moon. A quote from the cover of Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions by William David Compton: "Apollo 15 commander David R. Scott confirms Galileo's hypothesis that in the absense of air resistance all objects fall with the same velocity. A geologic hammer in Scott's right hand and a falcon feather in his left hand reached the surface of the moon at the same time (see chapter 13). The demonstration was performed before the television camera on the lunar roving vehicle, and no photographs were made." 
  • Sutton M-79; DaR M-088; Joseph ea p368.
  • Conant, "Harvard Case Histories in Experimental Science", Case 1: Robert Boyle's Experiments in Pneumatics. Experiment 40 About the falling, in the exhausted receiver, of a light body, fitted to have its motion visibly varied by a small resistance of the air.
    [describes an experiment in which Boyle contrived to drop a small cross made of four feathers in an evacuated container. This is presumably a fore runner of the coin and feather demonstration.
  • Millikan and Gale "A First Course in Physics" Revised Edition, 1913, page 89: That the air resistance is indeed the chief factor in the slowness of fall of feathers and other light objects can be shown by pumping the air out of a tube containing a feather and a coin (Fig.92). The more complete the exhaustion the more nearly do the feather and coin fall side by side when the tube is inverted. The air pump, however, was not invented until sixty years after Galileo's time.

Disclaimer

  • 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

Technicals

  • 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.