# Why earth rotates?

1. Apr 15, 2012

### woundedtiger4

Hi all,
I am not a physics student but I always used to think that earth rotates because of an inertia that occurred after big bang. But then someone told me that it's wrong. Now, I don't know that why earth rotates :(

2. Apr 15, 2012

### tiny-tim

hi woundedtiger4!

the earth rotates because (like all the other planets) it formed from the material that surrounded the sun, and that was rotating

i'll let someone else take up the story from there …​

3. Apr 15, 2012

### haruspex

If you take a cloud of particles in free fall moving in random directions, they will have a total linear momentum and a total angular momentum (relative to an observer).
As they coalesce, both will be conserved.

4. Apr 15, 2012

### Darken-Sol

then why do we always see the same side of the moon?

5. Apr 15, 2012

### turbo

The Moon is tidally locked to the Earth. It rotates, but the period of rotation is the same as its orbital period around the Earth, so we only get to see one side.

6. Apr 15, 2012

### D H

Staff Emeritus
Simple, but simply wrong. Google the term "angular momentum problem." Why the planets rotate is not a simple matter of conservation of angular momentum.

A better explanation, at least for the gas giants, is that protoplanets orbit slightly faster than the gas and dust in the portions of the protoplanetary disk near those planets. The orbital rate of a planet is $\sqrt{G(M_s+m_p)/r^3}$ while for a spec of dust it is just $\sqrt{GM_s/r^3}$. The protoplanet clears a path through the disk, and as it does so, it spirals in toward the nascent star. The planet encounters more material on its starward side as opposed to its outward side. This density gradient is what generates most of the planet's rotational angular momentum. It is not a simple matter of conservation of angular momentum.

Why the inner planets rotate is still problematic, and an even bigger problem is why the Sun is rotating so slowly.

The best answer to the OP's question is that the Earth is rotating now because (a) it was rotating shortly after it formed, and (b) it has only transferred some of that initial rotational angular momentum to the Moon's orbit.

7. Apr 15, 2012

### DaveC426913

To elaborate, the Moon was not always tidally locked. This is a common phenom that happens with moons of planets over their lifetime (billions of years), as the planets works to slow its moons' rotation. Many planets in the solar system have tidally locked moons.