h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There is one thing that have crossed my mind many times but I've never been told a good answer to this question.
How can the astronauts float around weightless in space nearby the earth while the earth's gravitation affects the moon??

They are both in orbit. The astronauts are just orbiting a bit closer.

There's plenty of gravity there, but when astronauts are 'weightless' it's only relative to their immediate surroundings. They are still being pulled by the Earth, the Moon, the Sun and even their spaceship, it's just that relatively the effect isn't noticeable.

Did you ever wonder why when astronauts are in orbit around the Earth they refer to is as free-fall? It's because they are gradually falling towards Earth, but they are doing it while also orbiting, like when water spins around a plughole. Eventually, they'll re-enter the atmosphere (anyone remember Skylab?) Because their craft is falling at the same rate, they get the effect of weightlessness.

If you ever take physics you will learn that gravitational pull between any two bodies is directly related to Mass. The moon and earth are incredibly massive, hence, the moon effects the earth and vice versa. A man is relatively massless, at least compared to the earth and the moon, and so his pull on the earth is very small, but still existent. Also, according to Newton, the astronaut is falling, at an incredible rate of verticle speed, but he's also traveling horizontally, at such a high rate of speed that by the time he would have hit earth, he's already gone so far that there is no longer earth below him, but gravity pulls him out of his linear travels, so that the earth is now below him again, etc., etc...

Im just wondering if I can accuse you of being tangential - but I dont think I'd get away with it.


;^)#

Eeek! All these questions! I'm gonna get smarter here whether I like it or not!

"Im just wondering if I can accuse you of being tangential - but I dont think
I'd get away with it. "

No you won't!

Thanks for the clarification Si.

;^)#

Ooooh! What's that shiny thing over there?

Easypeasy.

Technically they are not totally weightless but thats beside the point, you see Newtons law of gravity states that the force between two objects is proportional to the product of their mass and inversely proportional to the square of the distance between them. Basically gravity depends on the size of BOTH objects and the Distance between them.

Because astronauts are so small compared to the moon the effect is not great but the massive moon is affected and indeed affects earth(the tides of the sea).

Hope Ive answered the question, any more physics related Q's just ask as I have a brain full of (previously) useless knowledge.

EEek! Wells emcees quared.

OK Slarti-Brain-the-size-of-a-planet-bardfast, try this one. The Earth sucks. No, I mean gravity sucks. Well, OK, gravity pulls on our feeble frames and holds us to the earth. That gives us weight. Why then do we not weigh noticably less when the Moon is overhead and 'sucking' us up a bit. What about when there is an eclipse and the moon AND sun are overhead. I could lose tens of pounds here! Surely all I need to do is make sure I remain under the moons shadow! Must be better than dieting.

Same thing as was mentioned several times before: gravity is related to the mass of the objects. Human's aren't terribly massive, so the moon's (and sun's) effect on us is minimal.

But here's one that came to me the other night (while under the influence of a particularly nifty mind altering drug): If gravity has a stronger influence on more massive objects, then why do heavier objects fall towards the ground at the same rate as lighter ones? Hmmmm? Well, why? Makes no sense to me. Only thing I can think of is that they do fall at a different rate, just that the difference is so minimal that we don't notice it. (Like the effect of the moon on us.)

Nah, it's entirely reasonable: heavy objects are harder to get moving (more inertia). This *exactly cancels out* the fact that heavy objects are pulled down harder.

There's a very easy way to think about this so that it makes sense. Imagine two 1kg weights, falling right next to each other - almost touching. Now imagine two 1kg weights, falling joined to each other. Both sets of weights fall at the same speed, despite the fact that the seconds set of weights could be considered to be a heavier object.

Now imagine a big object falling... suddenly it splits into two pieces. Should the two pieces fall less slowly than they were falling when they were joined together? Nope. Same as above.

Basically, gravity acts on every tiny little bit of everything independently - it doesn't care about the whole object. So every object, independent of shape or size, is accelerated downward at exactly the same rate in the absense of other forces. (Assuming it isn't *too* big, in which case things get complicated due to the afore-mentioned inverse-square law).

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