Re: Gravity Inside A Cylinder
From: Steven Gray (sgray2_at_NOcfl.rr.comSPAM)
Date: 08/08/04
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Date: Sun, 08 Aug 2004 00:02:09 GMT
"Jaxtraw" <jaxtraw@nospamnobigfoot.com> wrote in
news:1091853310.18037.0@dyke.uk.clara.net:
> Specifically, suppose one had a hollow cylinder of finite length and
> perfectly circular cross-section, say, 1000m long and 10m in diameter,
> and constructed from something very massive, such as neutronium (kind
> of stuff one finds lying about :). We can mark the ends A and B, and
> call the centre O. Now suppose one has an object held inside the
> cylinder perhaps 250m from the centre point towards B. My guess is the
> following;
>
> 1) Because there is more of the cylinder's mass in the direction of A,
> the object would feel a gravitational pull towards A (or O, really)
> along the length of the cylinder.
Correct.
> 2) The object would *not* be attracted towards the walls of the
> cylinder for the same reason that an object inside a hollow sphere
> feels no gravity.
Correct, but only at the axis. In general there will be force toward the
wall at points off the axis.
> If we release the object, it will fall along the cylinder, accelerating
> until point O and then decelerating for another 250m, at which point it
> starts falling back along the cylinder and, ignoring air resistance
> etc, will oscillate indefinitely back and forth; at all times it will
> be in free fall.
>
> Is this right?
Assuming it starts on the axis, yes.
> Another thing; other than at the time when the object is released, am I
> right to think that it won't *feel* any acceleration? (Suppose it were
> a person, they'd not feel anything at all). I'm assuming it's
> effectively in orbit (freefall) around the cylinder's centre of mass,
> in an orbit with a minor axis length of 0.
>
> Am I right, or hopelessly wrong?
Neither. If the object is of non-zero size, then it will experience non-
zero tidal forces that it could "feel". Basically, though, you're
correct. In won't feel much in freefall.
> Also, while I'm at it, if the cylinder were infinitely long, would the
> object feel no gravitational pull at all? Or would it end up smeared
> all over the neutronium walls? :D
No pull.
-- Steve Gray sgray2@cfl.rr.com
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