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Feather Drop

We are all told at school that in a vacuum, all objects accelerate under gravity at the same rate, regardless of their mass, shape or size. This is something that I had never witnessed first hand. During the Apollo 15 mission, David Scott dropped a hammer and a feather on the moon to demonstrate that they would hit the ground simultaneously. The result of this demonstration looks rather strange though because of the low lunar gravity. It looks more like the hammer is falling abnormally slowly. I decided I wanted to witness this phenomenon first hand, so set up an experiment in the bell-jar vacuum rig.

As space is limited in the bell-jar, I decided to use a steel ball bearing rather than a hammer. The feather was held in place with a relay and the ball bearing was held by an electromagnet. The two were powered from the same supply so that they could be de-energized, simultaneously dropping the feather and the ball bearing.

The ball bearing weighs 67g and is 1 inch in diameter. The feather is less than 0.1g and does not register on my balance. I placed some crumpled up aluminium foil under the ball bearing to absorb the impact and prevent it from rolling or bouncing inside the bell-jar.

In the first part of the movie, the bell-jar is absent and the feather and ball bearing are dropped in normal atmospheric pressure. You will probably want to play it several times to see it properly. Note how the feather tilts as it falls due to air resistance. Clearly the ball bearing lands first.

In the second part, the bell-jar is in place and evacuated. The drop occurs four seconds into the movie. The feather and ball bearing are dropped and without air resistance, fall together.

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