Coin and Feather

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.

• For best results, hold the tube horizontally and shake it gently so that the 'coin' and '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 interefere with each other while falling. The recommended gentle shake in the horizontal position makes these occurrences very unlikely.

• You can show the effect of reduced pressure by allowing some air back into the tube and repeating the experiment. This will demonstrate that a good vacuum is essential for the success of this experiment.

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.