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*WHAT makes Earth ROTATE*? 
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#1
Jan3108, 08:04 PM

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....................so,what makes earth rotate....I believe I'm still in my *discovery*phase(normal for a 15 yrs old..)........sry if this is a newbish question....
I though looking on the internet..but so lazy and beside i will prob. not understand with they complicated terms and stuff they'll use to explain......o.0 also,i dun know if this makes sense or not,but can we assosiate dark matters with black holes........in someway,such as...dark matters can not be be seen and if im not wrong they don't even show any signs of energy(if they had,im sure they would produce some sort of gamma rays or w/e,=.=right?)..whereas black holes,we know they exist,yet...still many mysterious facts remain.... ???? so confused....o.0 hmmm any books recommendation...I was wondering to take some books and learn basics(very very very basic stuff...)about quantum mechanic(are they even useful in astrophysics?).. Thx:D 


#2
Jan3108, 08:55 PM

P: 253

hmmmm...............................
so.. 


#3
Jan3108, 09:14 PM

P: 36

When the solar system was formed, the matter had angular momentum. When the matter clumped together, the angular momentum from the individual pieces of matter had to be conserved, so things formed from this matter (like planets and even the sun itself) rotates.



#4
Jan3108, 09:54 PM

P: 15,319

*WHAT makes Earth ROTATE*?
Dark matter was hypothized as an explanation of why we see galaxies holding together while rotating too fast to do so. The only thing that seems to explain it is if the galaxies have more mass than we see. Not sure if that helps.



#5
Feb108, 07:51 PM

P: 253

woha...i dun think that i got wut you guys really are sayin...umm..then how come its still rotating...i mean,theres something...or else ..it wouldnt be rotating since all those years that passed........right???? o.0btw,im not even sure if i get this 'angular momentum'....and why did the matters had angular momentum when the solar system formed???
and..........hmmmmmm exactly how did we find out the existence of dark matters....? 


#6
Feb108, 09:17 PM

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Once you get it going, there's nothing to slow it down. (except for the moon of course) You really need to do the math: earth mass = 6e24kg earth radius = 6.371km rotational speed = 24 hours um..... I'm not near my calculus book at the moment ..... and the web seems to be a bit light on spherical gyro's today, 


#7
Feb108, 09:18 PM

P: 15,319

I think we would be a lot more willing to discuss this with you if you made an attempt to write intelligibly. It sort of feels like we're entertainnig a drunk.
In our everyday world, we are used to seeing spinning objects gradually slow down and stop  like a spinning top running down. But this is actually the exception to the rule. Newton's first law states that 'things in motion will tend to stay in motion unelss acted upon by an outside force'. Things slowing to a stop happens usually because of friction  in the case of the top, friction with the table it's on and the air. If the top were set spinning out in space, it would spin forever. So it is with the Earth. All that being said, the Earth is actually slowing down; it's just taking billions of years to do so. If you notice, the Moon has slowed to a stop (with respect to Earth at least) 


#8
Feb108, 09:40 PM

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in different words of course. 


#10
Feb108, 09:50 PM

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It's a question that's been going around for a day or two. 


#11
Feb108, 10:08 PM

P: 15,319

Formally, I only have a high school level of ecucation in math  I barely passed Calculus. Thirty five years of reading has alloowed me to understand much of physics, but I don't really have the skills to do the formulae. Pity. 


#12
Feb108, 11:25 PM

P: 225




#13
Feb208, 09:09 AM

Sci Advisor
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#14
Feb208, 09:51 AM

P: 253

well....let see if i really get it.
Basically,the earth rotation begun when this solar system formed and it had this angular momentum. It it is still rotating becuz of some newton law,since there is no force in space,theres nothing to stop it.right? o.0.......im very very slow at understanding :P However,there is something i am not understanding.How canthe moon be affecting earth rotation exactly...........???? 


#15
Feb208, 10:45 AM

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#16
Feb208, 10:55 AM

Mentor
P: 15,065

Any decent juniorlevel (college) classical dynamics text will cover the relationship between derivatives of vector quantities in rotating and nonrotating frames: [tex]\frac {d\mathbf q}{dt_{\text{fixed}}} = \frac {d\mathbf q}{dt_{\text{rot}}} + \mathbf \omega \times \mathbf q[/tex] where [itex]\mathbf q[/itex] is any vector quantity. Some call this the transport theorem (not to be confused with the Reynold's transport theorem), some (e.g., Marion, Classical Dynamics) don't bother to give this relationship a name. (Marion does a lousy job proving it, BTW). Apply this relationship to the angular momentum vector: [tex]\mathbf \tau = \frac {d\mathbf L}{dt_{\text{fixed}}} = \frac {d\mathbf L}{dt_{\text{rot}}} + \mathbf \omega \times \mathbf L = \frac {d}{dt}(\mathbf I \mathbf \omega) + \mathbf \omega \times (\mathbf I \mathbf \omega)[/tex] If the inertia tensor is constant in the rotating frame (and it is for an ideal rigid body), the inertia tensor comes out of the derivative. Solving for the time derivative of the body rate, [tex]\mathbf I\dot{\mathbf\omega} =\mathbf \tau  \mathbf \omega \times (\mathbf I \mathbf \omega)[/tex] These are Euler's equations, just written a lot more compactly than usual. 


#17
Feb208, 11:07 AM

P: 15,319

But the Moon causes lots of tiny forces of the Earth: it pulls on mountains and it sloshes water around. The net effect is friction  and a slowing of the Earth's rotation. Think about this: You turn off a ceiling fan (so that its spinning freely with no force or friction) and want to bring it to a stop quickly. It's much faster to bring it to a stop by grabbing its blades than by grabbing its smooth, circular body. Think of the blades like the mountains and oceans of Earth. 


#18
Feb208, 11:11 AM

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P: 15,065

The reason the Earth is slowing down is because of friction. The moon's gravity is a bit weaker on the side of the Earth away from the Moon than the side of the Earth facing the Moon. The tides result from this gradient in the gravitational acceleration. Friction between the rising and falling tides and the moreorless solid Earth are what are slowing the Earth's rotation rate down. EDIT: Dave C beat me do it. Dang. 


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