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Post by rick1776 on May 1, 2003 19:29:33 GMT -5
Revolving around the sun is not too bad. Keplers laws are being obeyed, conservation of angular momentum etc. But why does the earth actually rotate??
Looking forward to some answers. Its a tech question but not a motor one.
cheers
rick1776
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Post by Henrik on May 2, 2003 3:04:08 GMT -5
Not really sure why it rotates, but I think if id didn't, we would all be crushed by gravity!
Could it be that it spins in honor of the spinning entity Tora Takagi?
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Post by raptor22 on May 2, 2003 11:17:11 GMT -5
Good question!
This is not really understood and most astronomers seem to think that it is purely as a result of the intensity of the gavitational forces in the swirling mass of dust and and other matter from which our solar system and planets originated.
This energy has remained and due to the frictionless naute of space the planet should spin on it's axis for eternity.
Now if our solar system drifts through a nebula or dust cloud of sufficient density it will cause drag on the spinnng planets thus slowing their rotation.
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Post by CFF on May 2, 2003 13:27:50 GMT -5
Solar systems (planet or planets orbiting a central sun or suns) are all created in the same way: Giant (and we're talking HUGE here) clouds of interstellar gas and dust collapse to form stars (suns), which are then followed by accretion disks of cloud debris orbiting the central body *star*. This accretion disk is what becomes 'planets'.
Before all of this takes place, it's important to note that the giant cloud of interstellar gas/dust was rotating in space prior to the start of the solar system formation. This rotation is key to why the present solar system has planets that both revolve around a central body (the sun) as well as bodies that rotate on 'their axis'. As Rick & Raptor both alluded to, it's due to Conservation of energy. In this case the energy we're referring to is the angular momentum of the gas / dust cloud prior to it's collapsing upon itself to form a star and planets. This momentum (energy) cannot be destroyed, hence the rotation of planets on their axis is simply the maintenance (conservation) of the original angular momentum of the gas/dust cloud.
It's interesting to note that all of the bodies in our solar system 'rotate', including the sun. With the exception of Venus & Uranus, all the planets (and the sun) rotate counterclockwise when viewed from their northern pole. In the case of Venus & Uranus (retrograde rotations), it's hypothesized that collisons with a large asteroid or planetoid caused the axis of the planet to be changed, in Uranus's case to 90º normal, and in the case of Venus, 180º to normal (Venus rotates clockwise when viewed from it's north pole).
The gas giants (Jupiter, Saturn, Uranus & Neptune) rotate faster than the non-gaseous planets, again fullfilling the conservation of angular momentum therom (their mass far exceeds that of the non-gas planets, hence their angular momentum is much greater).
CFF
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Post by daSilva on May 2, 2003 14:12:08 GMT -5
Furthermore, the Earth actually wobbles on its axis.
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Post by rick1776 on May 3, 2003 9:27:56 GMT -5
CFF and Raptor22,
Good answers. The short answer is that no one knows for sure. It is hypothesised that since the primeval mass was swirling then as the planets grew, conservation of angular momentum continued the rotation. But this is a guess.
The interesting thing is that the rotation of the earth is slowing down. At inception a complete rotation took about 18 hours, we are now just short of 24 hours. Scary stuff!! Damn I need to adjust my watch again.
cheers
rick1776
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Post by rick1776 on May 3, 2003 9:34:23 GMT -5
In another life as a science teacher (I always found year 9 kids to be total arseholes, so I gave up the noble profession of teaching) I asked my year 11 class a bonus question in their physics test. I asked them if we took gravity as being 9.8m/s2 at the poles what would it be at the equator. Without doing the sums again I think the answer was about 9.77m/s2 due to centrefugal acceleration. Some kids actually got it right!!
cheers
rick1776
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Post by pabs on May 3, 2003 11:24:46 GMT -5
errr.....centrifugal acceleration? That doesn't even exist. What people call centrifugal acceleration is actually just an inertia effect and it doesn't even point radially out of a circular path.
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Post by Raptor22 on May 3, 2003 11:44:24 GMT -5
centrifugal acceleratoin = Angular acceleration ??
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Post by Mo on May 3, 2003 11:50:55 GMT -5
I have never thought of that, and the reason is that I have always found it simply natural... If we have atoms and molecules with particles spinning arond their nuclei, then the universe is sort of the same in a macro scale...
I read many years ago that it was because of a turtle... (I think from Sagan -RIP.) Loved that.
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Post by raptor22 on May 3, 2003 17:41:02 GMT -5
I have never thought of that, and the reason is that I have always found it simply natural... If we have atoms and molecules with particles spinning arond their nuclei, then the universe is sort of the same in a macro scale... well assuming you still think of atoms and electrons and other particles as little balls in orbit and not quanta then yes it is a prefectly acceptable model. The earths rotation has of course also been slowed by the numerous impacts of comets and large asterroids. Then the disttribution of mass along the surface will also influence it's rotation. Sit on a chair with a solid base but can swivel about a central axle and spin yourself round holding your arms close to your body. You will spin fast. now slowly extend your arms outward and you will start to slow. our mass has stayed the same, the torque on the shaft has stayed the same (minus the friction losses) but yuor rotation has slowed. Why? Easy your radius of gyration (or Sweetspot as the refer to it in raquet sports) has moved further out. Sam happens to the earth. Mountains increase in height, polar ice caps melt registributing the mass changing the RadOf Gyration and thus affecting rotational velocity. I hope i remember this correctly because that half course in astronomy was a longh time ago  |
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Post by pabs on May 3, 2003 18:05:54 GMT -5
Raptor,
Is radius of gyration the same thing as moment of inertia?
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Post by raptor22 on May 3, 2003 18:37:31 GMT -5
Pabs,
The Radius of Gyration of a Mass about a given axis is a distance k from the axis. At this distance k, an equivalent mass is thought of as a Point Mass. The moment of inertia of this Point Mass about the original axis is unchanged.
The relationship is that the square root of the moment of inertia is proportional the radius of gyration if th mass remains constant
or k2x = I/m
Where k is the raidius of gyration
I is the moment of Inertia
m = Mass
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Post by pabs on May 3, 2003 19:09:06 GMT -5
Gotcha...
It's just that I always thought about that problem as a Moment of Inertia thing, which is also correct since as k increases so does I, right?
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Post by raptor22 on May 3, 2003 19:28:28 GMT -5
Correct!
to increase Iyou have to square the distance k while the mass remains constant. The two are intimately linked. To increase the I with the same mass and energy you have to change the distance at which the mass i concentrated from the axis of rotation
so you could say that the k is the distance from the axis at which all the mass of the body is concentrated.
The two conceots are linked and lead to similar visualisation of the idea but they are in fact totally different. This can be seen from their units as well.
I=kg/m2
k=metres
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