Why does gravity make things spin?

[DaveC426913]

Well, there are theories that posit a rotating universe...

 

[Nyxie]

the tiniest of imbalances [like the gravity of the nearest star] will impart a preferred direction of travel.

This and an understanding of angular momentum sum it up based on textbooks/professors I know.

Also, think about why this spinning doesn't happen as much when you throw something on Earth. In space where we're far from strong gravitational influences (like stronger pull of gravity on Earth's surface) smaller gravitational forces have a greater effect since they aren't overpowered by larger ones.

An astronaut throwing a pencil won't give it a perfectly linear momentum in the first place anyway, since there's some curvature in the movement of the arm and it's reaaally unlikely they'll throw in a perfect straight line.

 

[Nyxie]

BTW why are people saying that we don't understand why this rotation happens??

 

[DaveC426913]

This and an understanding of angular momentum sum it up based on textbooks/professors I know.

Also, think about why this spinning doesn't happen as much when you throw something on Earth. In space where we're far from strong gravitational influences (like stronger pull of gravity on Earth's surface) smaller gravitational forces have a greater effect since they aren't overpowered by larger ones.

An astronaut throwing a pencil won't give it a perfectly linear momentum in the first place anyway, since there's some curvature in the movement of the arm and it's reaaally unlikely they'll throw in a perfect straight line.

Don't think you're really getting the idea here.

Take an arbitrary volume of space fileld with dust and gas, give the particles of dust and gas some arbitrary random motion.

As the dust and gas move under the influnece of their mutual gravity, they will fall toward each other. Their initial inertia will be preserved, so that, by the time they collect into clumps and solar systems, the clumps will have some of that spin, even while the whole system orbits.

 

[Nyxie]

As the dust and gas move under the influnece of their mutual gravity, they will fall toward each other. Their initial inertia will be preserved, so that, by the time they collect into clumps and solar systems, the clumps will have some of that spin, even while the whole system orbits.

OK, that explains things. But what about the astronaut and the pencil? Its inertia after being released/thrown is primarily determined by how it's thrown, right? so what else is it determined by?

 

[DaveC426913]

OK, that explains things. But what about the astronaut and the pencil? Its inertia after being released/thrown is primarily determined by how it's thrown, right? so what else is it determined by?

Well, yes. It's just that the pencil scenario doesn't really say much about why heavenly bodies spin.