Question about Gravity

Question

J.M.J

I understand that two objects pull upon each other with the same gravity, as the gravity equation gives one value for two masses: F = Gm₁m₂/r² . However, it sounds absurd to say that a tennis ball pulls on the earth just as much as the earth pulls on it. What am I misunderstanding?

Answer 1

Look at my video to get a detailed answer!

Answer 2

I wonder if you're getting stuck on the difference in scale of the mass of the tennis ball vs the mass of Earth. Also, don't be thrown off by the different rates of acceleration due to the force of gravity that the Earth vs the tennis ball would experience. The force of gravity F is the same for both masses, but the magnitude of acceleration is not. The relative mass of a tennis ball is so much smaller than that of the Earth that the effect is negligible, but yes, weird as it is, the tennis ball pulls back on the Earth. If you drop the tennis ball, it accelerates toward Earth at 9.81 m/s², and the Earth accelerates toward the tennis ball at some insanely small value. See if you can work that value out for yourself.

If that didn't help, think about this conceptually via the Earth-moon relationship. The moon also pulls back on the Earth, and at the risk of some oversimplification, our ocean tides are caused by this relationship. Or, you can imagine a universe with two stationary, non-rotating tennis balls of equal mass and size in space (with no other mass around them). They would also gravitate toward each other and bump into each other at a distance of approximately r/2 (r divided by two, not r squared).