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Suppose we have two stones, the first being lighter than the second. Release the two stones from a height to fall to Earth. Stone 2, being heavier than stone 1, falls more rapidly. If they are joined together, argues Galileo, then the combined object should fall at a speed somewhere between that of the light stone and that of the heavy stone since the light stone by falling more slowly will retard the speed of the heavier. But if we think of the two stones tied together as a single object, then Aristotle says it falls more rapidly than the heavy stone.

The question is how do the stones know if they are one object or two?

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  • $\begingroup$ You say that the heavier one falls more rapidly. Wrong! They accelerate at the same speed, thus fall at the same speed. This sing course neglecting air resistance (eg falling feather vs falling rock). $\endgroup$ – Gimelist Aug 1 '14 at 13:39
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    $\begingroup$ This is a Physics question... $\endgroup$ – Schism Aug 1 '14 at 18:43
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The two stones will accelerate at the same rate due to gravity. This is usually attributed to Galileo although it was known before that - eg. John Dee was aware of it and didn't think it a new revelation.

So if the two stones have the same shape and size - ie. so aerodynamic effects are identical, they will fall at the same acceleration and speed. There are plenty of experiments to demonstrate this, but perhaps one of the most well known was by one of the Apollo astronauts (the Moon being essentially airless has no aerodynamic effects). The result can also be derived from Newton's Inverse Square Law of Gravity and his Second Law of Motion.

If I were you, I would forget about Aristotle when it comes to trying to understand the world around us. He didn't have any concept of forces, inertia, and momentum as we would understand it today. His concept of forces was provably wrong with knowledge available to him at the time.

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