Nature & Beasts
Why does a coin and a feather fall at the same speed on the Moon?
- There is no air to slow the feather
- Gravity is weaker
- The feather is heavier there
The answer is There is no air to slow the feather. No air resistance - Apollo 15 did this with a hammer and a falcon feather.
Because the only thing that normally makes a feather fall slowly is air, and the Moon has none. Gravity accelerates everything equally regardless of mass - a heavier object is pulled harder and is correspondingly harder to move, and the two exactly cancel.
Commander David Scott did the demonstration on Apollo 15 in 1971, dropping a geological hammer and a falcon feather in front of a television camera. They hit the dust together. He said it proved Galileo right, which is a nice line and not quite the history: Galileo's leaning tower experiment is probably a story invented by his biographer, and he made the argument mainly by reasoning and by rolling balls down ramps.
The reasoning is the elegant part. Suppose heavy things fall faster; tie a heavy object to a light one; the light one should slow the heavy one down, yet together they weigh more and should fall faster still. The assumption contradicts itself, and no experiment is needed to see it. The equivalence has since been tested to about one part in a hundred trillion, because if it ever failed, general relativity would fail with it.