|If you drop a hammer and a feather together, which reaches the ground first?
On the Earth, it's the hammer, but is the reason only because of
Scientists even before
and tested this simple experiment and felt that without air resistance, all objects would fall the same way.
Galileo tested this principle himself and noted that two heavy balls of different masses reached the ground simultaneously, although many historians are skeptical that he did this experiment from
Leaning Tower of Pisa as folklore suggests.
A good place free of air resistance to test this equivalence principle is
Earth's Moon, and so in 1971,
Apollo 15 astronaut
dropped both a
hammer and a feather together toward the surface of the Moon.
Sure enough, just as scientists including Galileo and
Einstein would have predicted, they reached the
lunar surface at the same time.
equivalence principle states that the acceleration an object feels due to gravity does not depend on its mass, density, composition, color, shape, or anything else.
The equivalence principle is so important to modern physics that its depth and reach are still being
tested even today.
Apollo 15 Crew,