Malagasy Rain

I need some clarification as to why the planets rotate on their respective axes as they're orbiting the Sun. I'm pretty sure it's gravity but I want to be certain. Is it gravity?

Jesse

It's just conservation of angular momentum. If the sun suddenly vanished they'd all fly off on tangents, but they'd continue to rotate on their axes.

Malagasy Rain

Conservation of angular momentum. Care to elaborate?

Coragyps

Anything spinning will tend to remain spinning. A runaway truck wheel, for instance, will go a long, long way, and tear up all kinds of stuff while it travels. Friction and energy losses due to its rubber being not perfectly elastic finally make it stop turning - but something like the Earth has only the tiny amount of friction caused by tides to slow it down - and much of that loss gets transferred into moving the Moon into a wider orbit, which keeps the total angular momentum nearly constant.

The traditional illustration is a figure skater doing a spin - she starts with her arms extended, and spins rather slowly. But as she pulls her arms in, she speeds up. The amount of mass, its distance from the axis of rotation, and the speed of rotation all come together mathematically to give a constant (until you factor in friction).

In the case of the solar system, a very large, very slowly rotating cloud of gas and dust collapsed due to gravity to make the Sun surrounded by a disk, with the sun spinning pretty fast and the disk orbiting it more slowly. Chunks of dust in the disk bumped into each other, and stuck, in a fairly random fashion, while still doing their best to conserve their speed/distance/mass relation relative to the Sun. But each one's individual rotation on its own axis has to do with how the chunks happened to hit each other, so planets differ in rotational speed. With no significant friction, though, they keep spinning at nearly constant speed for many millions of years.

Young stars spin pretty fast, while our sun takes 25 days or so for one rotation. This slowdown is due to the "friction" of the Sun's magnetic field dragging through the solar wind - all the charged particles that it spews out all the time.

echidna

Ice skaters bring their arms in to spin faster & at the end of their spin throw their arms outward to slow the spin & break out. But arms-in spinning fast or arms-out spinning slow, the angular momentum is conserved (assuming the skater is a closed system).

Jesse

Well, are you familiar with the idea that objects move in straight lines at constant velocities indefinitely unless some external force (air resistance, for example) is applied to them? That's Newton's first law in a nutshell. Another way of stating this is that linear momentum, the object's total mass times the velocity vector of its center of mass, will stay the same in the absence of external forces, even if the object is made up of various components which interact and change each other's momentum--the total linear momentum of the object is "conserved".

The same sort of thing applies to rotation--in the absence of an external force applied in a way to slow down or speed up an object's rotation (force which affects rotation is called 'torque'), an object will keep rotating at the same rate forever. This can also be stated in terms of "conservation of angular momentum", with angular momentum technically defined as the product of the "moment of inertia" and the "angular velocity" (see the bottom part of this page for more details and links to further explanations).