Coin and Feather

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Equipment: A 6' long plexiglass tube containing a "coin" and a "feather". The tube is sealed and can be evacuated.

What it does: When the tube 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.

Concepts Demonstrated: The principle that, in the absence of air resistance, all objects at a given location fall with the same constant acceleration.

Classroom assembly:

  1. Start with air in the tube. Make sure the valve on the tube is open and the tube has been vented.
  2. Pick up the tube and hold it vertically so the "coin" and "feather" fall to the bottom.
  3. Repeat as necessary. Point out that the "feather" falls at a slower rate than the "coin".
  4. Attach the vacuum tube to the pump. Ensure the valve on the tube is open.
  5. Turn on the pump and let it pump until the pressure in the tube has reached a stable low value.
  6. Close the valve on the tube and turn off and diconnect the pump.
  7. Pick up the tube and hold it vertically so the "coin" and "feather" fall to the bottom.
  8. Repeat as necessary. Point out that the "coin" and "feather" fall at the same rate.



Cautions: 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.

Setup Time: Medium. 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.

Difficulty: Medium.

Visibility: Medium. The vacuum tube is large, but the "coin" and "feather" are difficult to see from the back of a large lecture hall.

Related demonstrations:

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.
Sutton M-79; DaR M-088; Joseph ea p368.
Some quotes may be found here.

Original Construction: 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.

Background: 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. We have the clip on videodisc, see references.

Disclaimer: All demonstrations are posted for the convenience and benefit of faculty and staff in the Department of Physics at Simon Fraser University and are not intended for outside use. The author(s) assume no responsibility or liability for the use of information contained on this site. Warnings and precautionary measures listed on this site assume normal operation of equipment and are not inclusive. Demonstrations may pose a significant hazard and can, in some instances, result in death; reasonable safety precautions must be taken. Demonstrations should be performed by qualified individuals only.

Prepared by Jeff Rudd, 1999
Revised by Laura Schmidt, May 2007

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