One dimension - Freefall of a feather and penny [video]
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This classic demonstration dramatically shows that all bodies in a gravitational field have the same acceleration in the absence of air resistance. A penny and feather are in a closed acrylic tube with a valve. The tube is attached to a frame that can be quickly inverted by hand about a horizontal pivot. With the valve open to the air, the tube is held vertical and then quickly inverted to show that, as expected, the penny falls much faster than the feather. A vacuum pump is then used to pump down the tube until the pump is heard to labor. The valve is closed, the pump is turned off, and the hose is detached from the tube. The tube is then quickly inverted as before. This time, however, the penny and feather are observed to reach the bottom at the same time, with the feather falling much faster now. The air resistance has been reduced to nearly zero. Finally, the valve is opened and the tube is rapidly inverted again, to repeat the initial demonstration. The gravitational force on a mass m is mg, where g is the acceleration due to gravity. In the absence of drag, Newton's 2nd law F = ma yields mg = ma, or a = g, which is independent of g. What is happening is that the force is proportional to the mass. However, for a given force, the acceleration is inversely proportional to the mass. The two effects thus exactly cancel in a vacuum.
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