M-D10: Coin and cotton falling simultaneously in air and in a vacuum.

PIRA: 1C20.10

Equipment: vacuum pump, long glass tube fitted to be evacuated in which there is a coin and a very light tuft of cotton (to replace the more traditional feather). The pump is on a cart in either Room 75, 73, or 71 and the glass tube with coin or cotton is in Set 8, Cabinet 2, Shelf 4.

Procedure: This is the classical experiment of "the guinea and the feather." The cotton tuft is superior to a feather because it is readily seen by the students at a distance. It is also more durable than a feather.

Open the cock to insure that the tube is full of air. Hold the cylindrical tube vertically and then suddenly reverse the upper and lower ends. The coin is heard to clink as it falls rapidly to the bottom, while the cotton floats gently down. Connect the tube to the vacuum pump and pump until the sound of pumping indicates a reasonably good vacuum. Turn the cock to the closed position and disconnect the pump. Repeat the previous experiment of overturning the tube. The coin will be heard to fall as before, but the cotton will be seen to fall almost as rapidly as the coin. It is best not to repeat more than is necessary to establish the point of the experiment as the bouncing coin beats the cotton into a wad which falls more rapidly in air than one would like. It is quite a chore to melt the wax, take off the fitting the tube and put in new cotton.

If you wish, you can discuss the problem of parachuting on the moon.

This demonstration is also available on the Mechanical Universe videodisk, which contains the NASA demonstration on the surface of the moon.