Register to reply

Why earth rotates?

by woundedtiger4
Tags: earth, rotates
Share this thread:
woundedtiger4
#1
Apr15-12, 08:02 AM
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 :(
Phys.Org News Partner Physics news on Phys.org
New study refines biological evolution model
Fiber optic light pipes in the retina do much more than simple image transfer
Production phase for LSST camera sensors nears
tiny-tim
#2
Apr15-12, 08:09 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
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
haruspex
#3
Apr15-12, 08:37 AM
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.

Darken-Sol
#4
Apr15-12, 08:57 AM
Darken-Sol's Avatar
P: 154
Why earth rotates?

Quote Quote by haruspex View Post
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?
turbo
#5
Apr15-12, 09:08 AM
PF Gold
turbo's Avatar
P: 7,363
Quote Quote by Darken-Sol View Post
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.
D H
#6
Apr15-12, 12:16 PM
Mentor
P: 15,053
Quote Quote by tiny-tim View Post
the earth rotates because (like all the other planets) it formed from the material that surrounded the sun, and that was rotating
Quote Quote by haruspex View Post
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 [itex]\sqrt{G(M_s+m_p)/r^3}[/itex] while for a spec of dust it is just [itex]\sqrt{GM_s/r^3}[/itex]. 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.
DaveC426913
#7
Apr15-12, 12:43 PM
DaveC426913's Avatar
P: 15,319
Quote Quote by turbo View Post
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.


Register to reply

Related Discussions
What is the reference frame the earth rotates relative to? Classical Physics 4
Why earth rotates around ? General Physics 45
Prove the Earth Rotates This Way? General Physics 21
Who said: however the Earth still rotates Astronomy & Astrophysics 9
Who rotates around whom? Astronomy & Astrophysics 8