The Earth’s gravity is strong enough to keep The Moon, an immense celestial body, from flying off into space. It is also weak enough to allow us to jump and throw things in the air. How does this work?

Question

Similarly, why does The Moon’s gravity affect the tides but won’t, for example, cause a paperclip to slide across a desk?

In: Physics

Answer 1

Well, you don’t fly off into space. You can barely jump like maybe the height of your body and then you “fall back to the Earth” pretty damn fast. So in a way the Earth’s gravity is strong enough to keep you on Earth, and weak enough to merely keep the Moon in orbit.

The force of gravity itself increases with mass, but decreases with the SQUARE of the distance, and the Moon, while “immense”, is also quite some distance away, compared to how close you are to the Earth.

But ultimately, tie a rock to a rope and see what force is required to “keep the rock from breaking the rope and flying away”. If you put the rock and the rope on the ground, the force required is 0. If you pick it up and start spinning it around above your head, you’ll have to hold the rope with some force. You’d have to spin very fast to actually have the rock break that rope, beyond what you can do with just your muscles.

So bottom line, the Moon doesn’t spin around the Earth all that fast. Gravity is enough to keep it there, like a rope would.