The experiment we know Galileo actually performed was an easier one to construct and control: he rolled balls of different masses down inclined planes. He was able to show that the balls accelerated in a uniform fashion, by an amount that depended on the angle of the plane but not on the masses of the balls. He then suggested that if we could trust this result all the way to planes that were inclined absolutely perpendicular to the floor, that would be exactly like dropping objects straight down, without a plane there at all. Therefore, he concluded, all masses would fall in a uniform way
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