# Why earth rotates?

by woundedtiger4
Tags: earth, rotates
 P: 176 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 :(
 Sci Advisor HW Helper Thanks P: 26,160 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 …
 Homework Sci Advisor HW Helper Thanks ∞ P: 9,643 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.
P: 154
Why earth rotates?

 Quote by 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.
then why do we always see the same side of the moon?
PF Gold
P: 7,363
 Quote by Darken-Sol then why do we always see the same side of the moon?
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.
Mentor
P: 15,053
 Quote by tiny-tim the earth rotates because (like all the other planets) it formed from the material that surrounded the sun, and that was rotating
 Quote by 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.
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.
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 Quote by 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.
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.

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