Thread: Gravity

Why do planets move?

Why shouldn't they?

Seriously, though, if they didn't, they wouldn't be around. The sun attracts massive bodies like planets. If they are not moving in elliptical orbits they are either a) just passing by, or b) on their way towards the sun. In neither of which cases they'd still be around after 4.5+ billion years.

The second reason is conservation of angular momentum. Like the way a spinning top stands on its tip while a non-spinning one falls over, the dust cloud that formed the solar system was spinning. And like the top, it kept spinning as long as nothing disturbed it. So the planets started out both spinning around their own axis and around the sun. And since they started out that way, and since there isn't friction, they stayed that way

Actually it's a lot more complicated than that, because 3+ body systems are chaotic in behavior, and to be stable orbits must match a couple of criteria. But the math gets rather involved, and I'd rather explain it at a more - ah - civilised time of day.

Why shouldn't they?

Newtons law of inertia right there.

Not why do just the planets move?, but the sun also, why does everything move in space? We believe we must apply a force to get things moving. And after awhile they'll slow down because of friction. Is the theory from the first sling shot of the big bang that everything is moving since or does no one know.

Weell, there's not a lot of friction in interplanetary space, so that answers your question of why they don't slow down.

Generally, though, there are two kinds of motion: Translational and rotational. Translational motion is relative in the sense that if you were put in a black box with no way to know what was outside the box, you'd have no way - litterally no way - to tell how fast you were moving. Rotational motion, OTOH, is absolute: If the black box of before were spinning on some axis, you'd most definitely notice.

This looks like a nice link, explaining some of the difference. I haven't looked it over in any thorough way, though. I looked through this link a while back, and it's pretty good - has a somewhat different style, though.

EDIT: One of the more curious consequences of the Coriolis force (this site has a nice movie illustrating it in practice) was experienced by the British Navy during the Atlantic War. They got on the recieving end of two nasty surprises: One was that France had sold Argentina a load of Exocet missiles, and the British were most definitely not prepared to go up against NATO-grade weapons.

The other nasty little surprise was that their own ballistic missiles didn't hit. This was because they had overlooked the simple little fact that ballistic missiles with ranges measured in thousands of kilometres are significantly effected by the Coriolis force. Now, the English missile techs knew that, and they'd built their flight paths to compensate... But they'd forgotten that the Coriolis force changes sign when you cross the equator :-) Must have been some rather red ears somewhere, when they discovered that...

Thanks for the links, they were interesting and easy to follow.