Fields

Gravitation
Newton's law of Universal Gravitation:

F = GMm/r 2

has the implication that the acceleration felt by a object in a gravitational field (which is equal to F/m, from Newton's second law of motion), can be written:

a = GM/r 2

As a result the acceleration of an object in free fall is independent of the mass of the object itself, and only depends on the mass of the object it is falling towards. Galileo was the first person to state something close to this, when he challenged the generally accepted Aristotelian view that heavier objects fell faster. Galileo instead proposed that a heavy ball would fall as fast as a small, lighter ball. While this can be tested on Earth quite easily as long as the effects of air resistance can be ignored (e.g. dropping a canon ball and a marble over a short distance), the Apollo 15 astronauts tested the theory on the Moon (where there is no air) by dropping a hammer and a feather at the same time:

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