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How do people know the mass of the earth? Do they put it on a scale or something? No.
The answer is Physics. Now in high school, you've probably heard of Newton's law of gravitation:
F=G times m1 times m2 over r square. And you also know that the earth attracts you everywhere,
but the sum of all the force would go toward the center of the earth. Now suppose your
mass is small m, the earth's radius is R, and its mass is big M. So according to Newton's
law of gravitation, F equal to G times small m, times big M, over R square, and according
to Newton's second law, equal to small m times small g. Easy, right? Now Cavendish had found
out big G 300 years ago, so we don't need to care about that, and we also had found
out small g is equal to 9.8 meter per second square. Also, our mass is canceled out. The
only thing left is the earth's radius? How to calculate it? Surprisingly, Earth's radius
had been found two thousand years ago in Ancient Greece, but that something we will talk about
later. So earth's radius is also known, the only thing left is the earth's mass, which
is calculate to be approximately 6 times 10 to the 24 kg. And that's how we know the mass
of the earth.