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February 11, 2022 04:44 pm

The Greatest Physics Demo of All Time Happened on the Moon

This true story of a hammer, a feather, the Apollo 15 mission, and the answers to humanity's oldest questions about how stuff falls. From a report: Does a falling object move at a constant speed, or does it speed up? If you drop a heavy object and a light one at the same time, which will fall faster? The great thing about these two questions is that you can ask pretty much anyone and they will have an answer -- even if they are actually wrong. The even greater thing is that it's fairly simple to determine the answers experimentally. [...] OK, but what about dropping a rock and feather -- doesn't the rock hit first? Usually, the answer is yes. But let's replace the rock with a hammer and then just take a change of scenery and move the experiment to the moon. This is exactly what happened during the Apollo 15 lunar mission in 1971. Commander David Scott took a hammer and an eagle feather and dropped them onto the lunar regolith. Here's what happened: The feather and the hammer hit the ground at the same time. Why did it happen? First, it is indeed true that even on the moon there is a greater gravitational force on the hammer than the feather. We can calculate this gravitational force as the product of mass (m in kilograms) and the gravitational field (g in newtons per kilogram). On the surface of the moon, the gravitational field has a value of 1.6 N/kg. If you put this expression in for the net force on a falling object, it looks like this: Fnet = - mg = ma; a = -g. Since both the gravitational force and the acceleration depend on the same mass, it's on both sides of the equation and cancels. That leaves an acceleration of -g. The hammer and the feather fall down with identical motions and hit the ground at the same time. So, what's different about dropping something on the moon versus on Earth? Yes, there is a different gravitational weight on the moon -- but that's not the issue. It's the lack of air that makes the difference. Remember that Newton's second law is a relationship between the net force and the acceleration. If you drop a feather on the surface of the Earth, there are two forces acting on it. First, there is the downward-pulling gravitational force that is equal to the product of mass and the gravitational field. Second, there is an upward-pushing force due to the interaction with the air, which we often call air drag. This air drag force depends on several things, but the important ones are the object's speed and the size of the object. [...]

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