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Q & A: Two-Way Gravity

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Most recent answer: 10/22/2007
Q:
Isnít it true that objects of differing mass actually do NOT accelerate towards the Earth, or any other mass, at the same rate, instead it is based upon the mass of the falling object? All these school-kid descriptions of falling bodies are wrong at the fundamental level, bodies do not fall to the earth but they are instead attracted to the Earth just as the Earth is attracted to them. An elephant WILL fall faster than a feather, even when put in a vacuum, because gravity is not a one-way force. True, right?
- Mike (age 22)
Ames, Iowa
A:
Interesting point. If you use a standard frame in which to measure accelerations (say the average position of all the visible stars), then the acceleration toward the Earth does not depend on the mass of the falling object, at least so long as Newton's gravity is a decent approximation.
 However, as you point out, there's another acceleration going on- the small acceleration of the Earth toward the elephant. That means that, even ignoring air resistance, the Earth and elephant would collide a little sooner than the Earth and feather  would. In that sense (basically measuring accelerations relative to the Earth), the elephant falls faster.

Mike W.

(published on 10/22/2007)

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