Gravity!

[map08]

Hello! I was wondering if someone could explain a couple of things that I can't work out. The first is why do the planets all spin the same way - is it a physical law? I am aware that one of them - neptune, I think - spins in the opposite direction due to a collision, but I think I read that it is slowing down. The second question is what would happen, theoretically, if a human fell down a hole bored right through the earth's core, and continuing to the other side. Would they slow as they reached the core and eventually stop, or would they slow after they reached the core, and have an elastic effect centered on the core? Thank you for your time!

[WaldoMcFroog]

Hi! The planets all spin in the same plane because they formed out of a gas cloud. Spin is determined by the degree of rotation of the original cloud, but in a not so obvious way. As a rotating cloud condenses it gets hotter and spins faster. It's a basic law of physics to maintain the same momentum, if no outside forces are acting which was the case. But now it's called angular momentum, since it's motion of gas (and some dust, it turns out) is more-or-less along circles and not along straight lines. So all that was needed was a big cloud of gas and dust spinning somewhat slowly to give a small fast spinning planet after gravitational condensing. Notice that the planets are nearly spherical in shape, including Earth. Well, as the parts of the big cloud condense and heat up they go into a molten state, making it easy for the internal gravitational forces acting uniformly in each (in a spherical sense) to make a lot of spinning spheres. In physics we say it's a minimum energy state, which nature always strives for. This can be scientifically reproduced under lab conditions by the way. You're right about one of the planets being out of sync, and the cuase being a collision, but it's Uranus not Neptune, and it's at almost 90 degrees to the others. The spin of all the planets is slowing down - back in Jurassic times, the coral records show that the day was only about 16 hours long. I wouldn't expect to see much difference in your life time though!! :-) As for your other question, apart from the fact the person would die (obviously) and would be consumed long before it reached the centre, any body capable of surviving to the centre would stop at the core.

[map08]

Thankyou! I'm intrigued by the evidence that the planets are slowing down - you mentioned that the momentum continues as long as there is no force to slow it down. If the planets are slowing down, then what force is this?

[Squirrel]

Just as a totally wild guess, if it turned out that photons had a tiny mass, the effect of trillions of these being absorbed by the Earth every second, and less being emitted into space, would result in the Earth getting heavier...and slowing down maybe? Like I said - a wild guess.