# Rotation of earth

#### alchemist

what keeps the Earth spinning? or more exactly, what drives the planets in our solar system to spin and rotate around the sun? what will happen if earth stop spinning?

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#### russ_watters

Mentor
Newton's first law: an object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an oustide force. So once in motion, no driving force is needed to keep the planet spinning and orbiting.

#### Phobos

Staff Emeritus
Gold Member
Welcome to Physics Forums, alchemist!

Originally posted by alchemist
what keeps the Earth spinning?
Like Russ said, inertia. Space is an ultra-low friction environment. There's not much to slow it down (although it is slowing). The material that came together to form the Earth had a net spin. Plus, collisions by protoplanets, etc. can add energy to the spin. If you think about it, it would be surprising if something like a planet didn't spin.

or more exactly, what drives the planets in our solar system to spin and rotate around the sun?
Spinning (rotation) - see above.
Orbiting the sun (revolution) - A balancing of the Earth's (or other planets/objects) velocity (direction + speed) vs. gravity with respect to the sun. Any object that is stationary with respect to the sun would be pulled into the sun due to gravity (just like any object falls to the ground on Earth). With movement that is not directly toward the sun, there is the potential to miss hitting the sun. With the right direction and speed, you get a stable orbit around the sun. Too fast, and you spin off into space. Too slow, and you spiral into the sun.

Where did it all come from? The solar system was formed from a nebula of material that collapsed under its own gravity. It came together asymmetrically (on local scales) and resulted in a net spin.

what will happen if earth stop spinning?
With respect to what? To the sun? If the Earth became gravitationally locked with the sun, then one side would fry and the other would freeze. If this happened slowly, then the change would be slow. If it happened in an instant (magically), then the entire surface of the Earth would be destroyed (think of the effect on you when a car stops suddenly...now consider that the Earth is spinning at about 1,000 mph at the equator).

#### alchemist

sun and stars

if thats the case does it mean that the sun is moving along too? is it spinning too? is this the case for the other stars in the universe?

#### LURCH

Yes, the Sun also spins. Being gasseous, it's rotation is not uniform but fluid, with the equator spinning at a different speed than the poles, with one revolution taking about 25 Earth-days at the equator, and about 35 at the poles.

The Sun is also moving in relation to the rest of the Milky Way gallaxy. As you probably know, our gallaxy is shape like a big whirlpool, a big flat disc with streaks forming a spiral pattern toward the center. Our solar system is near the outside of that disc, on the edge of one of the streaks (or spriral arms). The whole disc is spinning just like the planets (and the Sun) spin on their axes. As the planets orbit the Sun, which is the center of the Solar system, so the Sun and other stars orbit the center of the gallaxy (which is probably a really huge black hole). I've never thought to check and see if the local cluster of gallaxies also rotates, but I'd be very suprised if it doesn't; rotation is a very prevalant theme in this cosmos.

#### Loren Booda

Which causes the Earth to spindown more: friction with its molten core and atmosphere, or tidal friction from the Moon?

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