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

You are thinking of gravity like it’s a giant muscular arm pulling at something. If that was the case, pulling a massive object would have less of an impact than pulling something light. If that’s how gravity worked, the force to keep the moon in orbit would flatten everyone in Earth.

In reality, Gravity is just a constant speed change. It accelerates all objects who are the same distance from the center at the exact same rate. If you shoot a bullet straight up in the air, it might leave the barrel at a speed of 700 meters per second. It’s speed relative to you will decelerate to 690 meters per second by the second second. 10 seconds in, that’s 600 meters per second now. In reality, the bullet slows down a lot faster because of air resistance, but, in a vacuum, a 5 ton boulder and a a bullet weighing a fraction of an ounce would decelerate at the same speed.

The reason for this is that Gravity isn’t necessarily a force. We calculate it as such because it behaves like one, but that’s just a short cut. Gravity alters space time so that the path of the object is altered. This is why the mass of the object is irrelevant (I mean it matters because the object also has gravity which also alters space time, but those are overcomplications).