Jokes and Riddles Centre

[Suicide Junkie]

Because it is very unlikely that all the particles comprising the planet had velocities that would cancel each other out to zero when they came together.

[narf poit chez BOOM]

or, so we would have day and night.

[Ruatha]

Originally posted by Suicide Junkie:
Because it is very unlikely that all the particles comprising the planet had velocities that would cancel each other out to zero when they came together.

But to have such a fast spin as most planets have most particles would hav eto have the same general direction, or? (Why do stars spin in the galaxy? I know what keeps them from flying away but why do they spin?)

(It doesn't seem to be any answer, most belive it's from the big bang but it seems there are no good theory as to why things spins (planets arund their axis, all stars in the same direction around the galaxy, etc))

[Loser]

Actually the particles, orbiting a larger mass, would be traveling at different speeds based on how far away they were from the mass they were orbiting. As they clump up, they must travel at different speeds, and angles to each other. Then, once they are starting to form a body, there will be many bodies clumping together, pounding and knocking each other around. Since the bodies do not intersect through their collative centers of mass the impact would necessarily impart spin.

As for galaxies, they must spin or all matter in the galaxy would fall into the center. The orbital motion is all that keeps them apart from each other.

[Suicide Junkie]

Take an overall cloud of particles that may form a planet.
That cloud has some angular momentum. For material orbiting a star, the stuff closer in will be moving faster relative to the stuff farther away. (Since it is orbiting the star)

As the planet forms, the huge starting volume compresses down to a relatively puny ball of rock/ice/gas giant.

Since angular momentum is conserved, and the body is now smaller, the planet will be spinning quite fast.
Think diving or figure skating or merry-go-rounds. When you move the mass towards the center the spin rate increases, conserving angular momentum.
Imagine a merry-go round with a radius of a few million kilometers, collecting 6x10²⁴kg of rock and drawing it down into a ball only 6378 km in radius.

Even a small net spin will be magnified by a huge amount as the body collapses.
Bits spinning too fast may get thrown off, and slow bits will sap energy from the medium and fast bits.

[ August 21, 2003, 16:58: Message edited by: Suicide Junkie ]

[geoschmo]

Originally posted by Ruatha:
But to have such a fast spin as most planets have most particles would hav eto have the same general direction, or? (Why do stars spin in the galaxy? I know what keeps them from flying away but why do they spin?)

(It doesn't seem to be any answer, most belive it's from the big bang but it seems there are no good theory as to why things spins (planets arund their axis, all stars in the same direction around the galaxy, etc))

The answer is if they didn't, they wouldn't.

That seems simplistic and counter intuitive, but to understand it you have to look at it backwards. If there were things in the solar system that didn't revolve around the sun in a particular way then over time they would fall into the sun or another planet, or be flung off out from the system entirely. Early on there were very likely many objects of varying size that were going all about in a chaotic manner. Over time all those were eliminated and what we are left with is only those things that follow a few stable orbits. Plus some small debris of course. The galaxy is the same principle as the solar system on a larger scale. It's not that something is making them revolve a certain way, it's that over time things that don't revolve a certain way get eliminated.

As far as why planets spin on their axis, I believe most of that comes not from the impact of the debris forming the planet, but from conservation of momentum of orbiting bodies. A body orbiting another will pick up a spin from this process. It's compicated orbital mechanic stuff that I don't fully understand, but I think that is the jist of it.

[Suicide Junkie]

Jupiter is likely responsible for cleaning up a lot of the garbage floating around in the system.
Things passing in front of the planet get their orbital momentum sapped and fall inwards, while things passing behind the planet (like the voyager probes) get whipped out of the system.

[Loser]

I believe, Junkie, that you are right and I am not.

Impacts from lower orbits would be traveling at higher speeds, relative to the forming body, causing it to spin against it's orbit. Impacts from higher orbits would actually strike the leading face of the forming body, as the forming body has a faster orbit, again causing it to spin against its orbit. These examples assume all orbit are circular, but I'm thinking it will all average out that way, anyway.

If, however, the forming body is a cloud of sorts, it ought to be tide-locked to the star. In this case one face of the cloud will always be presented to the star, and thus the cloud will be spinning with its orbit. As the cloud collapses it will spin fast, with its orbit.

We'll just leave Venus and Uranus out of this, and say that all reasonable planets spin with their orbits. It's got to be the cloud thing.

[ August 21, 2003, 17:44: Message edited by: Loser ]

[Wardad]

Sigh, I want the jokes and riddles back....

[Loser]

Originally posted by Wardad:
Sigh, I want the jokes and riddles back....

Bring them. I like them. You seem to bring more than anyone else, although I could have been mistaken, and they were always funny.

[ August 21, 2003, 18:40: Message edited by: Loser ]