ERICH SCHREMPP / SCIENCE PHOTO LIBRARY ERICH SCHREMPP / SCIENCE PHOTO LIBRARY
Gravity experiment. Composite time-lapse image of a feather and a ball falling at the same rate in a vacuum, illustrating the concept of gravity. Any two objects of different weight (mass) accelerate at identical rates at the same gravitational potential. Therefore a feather and a ball when dropped together should fall at the same speed. The fact that they do not on Earth is due solely to air resistance which is eliminated in a vacuum. During the Apollo Moon flights one of the astronauts successfully demonstrated this principle by taking advantage of the lack of atmosphere on the Moon. The feather and hammer he dropped fell at the same rate.
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