How would gravity break a rock?

Answer 1

First off the idea that might sound very convincing is to simply drop the rock from a high altitude and let it shatter itself on the ground. This however is not really gravity breaking a rock, because the rock is kept together by molecular bonds, thus by electromagnetism, and while gravity is providing energy the ultimate disintegration of said rock is also due to electromagnetism (by molecular bonds from the ground smashing into those of the rock).

For a better idea we need to enhance the size of the rock a bit (it will work with a small rock, but it will look very silly and the environment needs to be more extreme). So let's grab a Moon.

Imagine a Moon in orbit around some planet. Now there is a way to break the Moon using gravity using an effect called tidal forces. You see, the Moon is generally kept together by the force of gravity and if we somehow could counteract this force we would shatter the Moon.

Tidal forces work in the following way. Imagine the side of the Moon closer to the planet. Because it is closer to the planet if will feel a larger gravitational pull than the opposite side of the Moon (which being further away, will feel a lesser pull). The difference in these forces acts to shatter the Moon. The difference need only be enough so that the Moon's own gravity is no longer enough to keep it together.

This is how the rings around varies planets were formed. A Moon wandered too close to the planet and it got splintered by tidal forces, leaving only small chuncks that circle the planet and formed the rings.

We ourselves are also subject to tidal forces, our head (usually) is further away from the Earth than our feet, so they feel a different strength of the gravitational force.

Why are we not shattered? Well firstly because we are very small compared to a Moon! Therefore the difference in force is also small. Secondly we are not held together by our own gravity, but by molecular bonds and as such electromagnetism. Luckily electromagnetism is far far far more powerful than gravity!

This is also the reason why we didn't use a small rock. The planet would have had to be extremely heavy and dense (so we can get close) for it to work.