OT: I'm confused about gravity

[Fredxx]

Is that like saying "if I jump off a block of flats it won't hurt"?

[Bill Wright]

Exactly. So all the molecules around me in every direction except straight down; the garden shed, Emley Moor mast, the potatoes in the stew the woman up the road is making, Ben Nevis, the fly on the ceiling above my head, are all pulling me towards them. So how can their gravitational force be pulling me towards the centre of the earth? It can't do both, and why should it? This means that the force of gravity as experienced by me (who is sitting here on the surface of the Earth -- well, just above it) is less that that of someone who is out in space with the whole earth pulling him in almost exactly the same direction, taking distance into account. I agree it won't be much different, because the overwhelming total of the forces will be downwards, but there will be a small difference. To someone miles away in space the gravitational force exerted by the fly on my ceiling will be in the same direction as all the other gravitational forces from everything else in and on the Earth. But to me it will be in the opposite direction. So I will be a bit lighter than I would be if I was in space, taking distance into account.

Bill

[Norman Wells]

Hitting the ground though ...

[Bill Wright]

So if I was at sea level high ground wouldn't contribute?

Bill

[Norman Wells]

No, it contributes even more. That's why you float in the sea.

[The Natural Philosopher]

[Bill Wright]

So under what circumstances would that be or not be the case? Would it be the case if (a) the G & T and ceiling lamp were on an aeroplane flying above the earth (b) the G & T etc was on an aeroplane that was on on the ground (c) the G & T etc was in orbit around the earth (d) the G & T etc was in a close flyby of the earth but was not orbiting around it.

Bill

[Bill Wright]

So Mt Everest would pull me at right angles to the rest of the Earth's total gravity, but if I was up in space it would pull be the same way because the centre of the Earth and the mountain would be in the same direction.

Gravitational force of the whole planet except Mt Everest + Gravitational force of Mt Everest = More than gravitational force of the whole planet except Mt Everest.

QED! And I said I couldn't do maths!

Bill

[Bill Wright]

I've heard it suggested, yes.

Bill

[Bill Wright]

Maybe not all that small. I've installed a lot of overhead catenary wires in my long and boring life. You'd be surprised how hard it is to get them straight with no dip. In fact you can't. No matter how much you tighten them they still droop in the middle. It's very obvious when your eye is near one end.

It's the same with string when you're trying to mark a line. It droops a lot.

Bill

[The Natural Philosopher]

all of the above

[Indy Jess John]

Not when the string is vertical ;-)

Jim

[Indy Jess John]

There was one turtle. In all the Pratchett books I read, there was a single turtle.

Jim

[Max Demian]

I don't remember doing that. They always solemnly printed the value for g on physics exam papers which rather annoyed me as it's unlikely I would forget such a thing whereas I was expected to remember all the formulae.

It might encourage you to think there is a logical reason for it rather than being an (approximate) coincidence.

You mean real pi, or another coincidence?

[Tim Streater]

Look, this whole thread started with gravity. The calculus bit comes in if you want to compute what gravitational field you experience as you go down a mine. Also to prove that *above* the Earth's surface, you can consider the Earth's gravity as if all its mass were concentrated at the centre, and then just use inverse square to see what you experience as you get further away.

[Max Demian]

How do you know what he's thinking?

Verne's story doesn't depict a hollow Earth, just a big cavern with a sea and a luminous ceiling to light it. And his explorers don't get anywhere near the centre of the Earth, despite the title.

[Tim Streater]

Well, theorists want to reduce it to a simple formula. So they have an idea and see whether that explains everything known so far. Then they start using the idea to make predictions. If *those* pan out, they have a workable theory.

Newton did that with gravity and it's still good enough for navigating around the solar system. It fails near (very) large masses like the Sun but then only when measuring something over a long period of time, like how Mercury's orbit changes. For that you need friend Albert.

[Tim Streater]

AFAIK, you cannot renounce British citizenship.

[Tim Streater]

[Tim Streater]

A catenary.