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Re: Spin......?????

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Posted by Alexander on February 28, 2001 04:35:39 UTC

from my text book in 1st grade to the introduction of "the brief history of time" by carl sagan....
i have known that celestial bodies show spin around their own axis including earth,moon,sun and even the mighty black holes in the galactic centres...and even somehting like human being would be sent spinning off into the space if the gravity of earth is switched off somehow....
my question is :
why do bodies show spin????factors and conditions affecting this spin???? and of course what will happen if bodies dont show spin abt their own axis..?????}

Macroscopic bodies do not have to "show" or exibit spin (rotation). Some spin, some not. Some more, some less. Some rotate in the direction of Polaris, some in Orion, some in other. It is like motion - some objects move north, some down, some stay.

The reason most selestial bodies rotate in some or other direction is history of their formation: if the bunch of hydrogen they formed from had slight asymmetric motion (thus non-zero angular momentum versus center of mass), this motion conserves during gravitational contraction and we have a spinning star (the smaller - the faster spin to keep the same angular momentum L=Iw, where I is the moment of inertia, w-vector of angular velocity (I=mr^2 for point mass m at distance r from the axis of rotation).

So different stars have different spin. Different galaxies too - you may see them oriented randomly. Due to collisions or gravitational interactions bodies can exchange or share their angular momentum, but total momentum always conserves (say, galaxies can precess in gravitational field of each other, but when you add all vectors of all L to get total L, it stays the same.

We on Earth spin with Earth as 1 revolution over 24 hours, plus one over 1 year, plus one over 26000 years (Earth precession) plus one over 2x10^8 years (galactic year). Thus, if you turn Earth gravity off and we all fly in space, we will continue to rotate making about one revolution in 24 hours.

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