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

I really want to try to explain this in a bit more complex way that really made it all click on my head. Hopefully everyone can follow along.

So to start with, we should note that there are two forces that act on you: gravity pulling you down, and Earth under you, refusing to let you in and therefore pushing you up. A net result is, you stay roughly still.

So, to understand gravity better, lets erase Earth with its upward pulling, space-taking, ball-resembling properties and just leave the gravitational field. Basically, we make it so that you can freely fall through Earth unaffected by anything other than its gravity, to better understand gravity.

Now, for you this would mean you start falling. But once you are at or near the center of Earth, it’s not like you stop. Your momentum now keeps carrying you away from Earths core, and you emerge from the other side at about the same height as you started with, before gravity finally wins and manages to turn you around and make you fall back towards Earths core.

So when you think about path you trace, you notice it would resemble a line, or perhaps an ellipse if you started with sideways motion. And now to high school physics part of this, you notice that all you’re doing is exchange gravitational potential energy to kinetic energy and back. Your total energy stays the same always.

And this means, without using energy to change your trajectory, you could not reduce this total energy you have and stay, on average, closer to Earths core. You make flybys but cannot stop going away again.

And you notice, for Moon these scenarios, with fall-through Earth and real Earth, are identical. Moon just keeps falling but has enough sideways motion that instead of huge yoyo motion, it does something more resembling circular orbit.

And to go back to the example of you jumping, that can be seen as you adding a tiny extra bit of energy to your orbit. But on real Earth, this orbit again ends right as you smash back onto Earths surface.