Eli5 on why do planets spin?
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Have you ever played billiards? Unless you hit a ball perfectly dead in the center it’ll have “English,” or spin. This is because the force from the cue ball or cue stick goes through and around the other ball’s center of gravity. If it goes dead center there’s nothing for the force to go around so the ball will slide straight instead of rolling or spinning.
All the rocks and asteroids and things in space have a LOOOONNNG time to hit each other imperfectly and off center, so they spin. Planets are just big collections of lots of space rocks (or gasses), but the forces are the same. They’ve had long enough that they bump each other enough to self-organize into the same plane with the same spin.
Conservation of angular momentum inherited from the spinning Sun. https://youtu.be/Yhtr2hbg9Rs
Inside the vacuum of space, before a solar system resembles a solar system, it is just a massive cloud of dust. This dust has one force acting upon it, gravity. Gravity pulls this dust towards itself, as it does, the whole thing starts to spin.
A reasonable way to think of this is with the plastic sheet demonstration that is used to show how mass bends spacetime.
Stretch a sheet out by all 4 corners, this sheet represents the fabric of space. Now throw a bunch of marbles at it, this represents your gas cloud. Now there are other forces here as the earth’s gravity has an effect, as well as the friction from the sheet, but when you toss a bunch of marbles at it, they first follow the curvature made by other masses on the sheet. This curvature causes the marbles to spin around each other. On the sheet, it stops quickly due to friction, but in space where friction is basically zero, the gas keeps rotating. This eventually gathers enough matter to form spinning rocks and spinning planets.
Basically, any slight force applied to something in space will continue for a very long time until acted upon with equal (or greater) force in the opposite direction. During the formation of a planet, gravity is the force acting upon the dust cloud and the effect of gravity causes the spin. This spin just carries on, in practical terms, indefinitely.
The real question is what could make them not spin. When an object is flying passed a mass (protoplanet) in space, it keeps whatever momentum it had as it approached the mass. The gravity of the mass might just bend the trajectory of the object as it whizzes by, but the object may be close enough and slow enough to be “captured” by the gravity well of the larger mass, causing it’s trajectory to be bent so much it goes into orbit around the mass, still maintaining it’s momentum (and thereby in turn “pulling” on the mass to make it spin). Or, more likely, the orbit is short-lived, and “decays” as the object circles around the mass, eventually crashing into it. The object isn’t really “falling”, it’s just getting closer to the mass as it continues “forward” under its original momentum. Every part of every planet started this way, as clouds of dust coalesced around their center of mass and crashed into each other to form a single massive object we call a planet. So planets start out spinning, and there’s nothing that can stop them.
This explanation ignores tidal forces, too complicated to explain and not large enough to matter in this context. The only real confounding issue is the randomness of the original momentum of the object, which theoretically could oppose rather than add to the rotation of the planetary mass. But solar systems form the same way as planets (coalescing clouds of material) so the whole system is “rotating”, meaning that the planets are orbiting the star (center of mass). Since the planet is moving through space as well as spinning, any object that is captured will add slightly more spin if it’s trajectory matches up with the planet’s orbital motion and will subtract slightly less spin if it’s trajectory is opposite or orthogonal, so statistically the spin of the mass will be maintained for essentially as long as the planet exists.
Because they don’t get dizzy from it. Why if people were the same way you’d have to tie people down just to stop them from spinning. If you’ve ever been around kids they love spinning. We were born to spin but society says we can’t do it.
The reason depression is so rampant right now can directly be linked to the lack of spinning in your life.
I’ve been fired from every job I’ve ever had because I insisted that I be allowed multiple spin breaks everyday. They would say things like “no one need to spin” or “spinning is for kids” but I just get the urges and they must be fulfilled. Everyone starts that way but is beat down. But you can beat the system and be an example and spin your little heart out everyday. The more people around to see the better. Stick it to standing still lobby and fight the good fight. Be like the planets and be free.
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