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Re: OT: Ringworld

From:Lars Henrik Mathiesen <thorinn@...>
Date:Tuesday, December 4, 2001, 18:14
> Date: Tue, 4 Dec 2001 09:44:39 -0800 > From: Anton Sherwood <bronto@...> > > Lars Henrik Mathiesen wrote: > > The sphere or ring itself will not exert any gravitational attraction > > on matter inside them --- > > True for the sphere, false for the ring, as ought to be obvious from the > informal geometric argument:
Right... I thought I had an argument from rotational symmetry that the ring case was like the sphere, but I see where it breaks down now. [snip clever argument for the sphere case]
> See for the contours of the ring's > g-field (actually 200 point masses on a circle). The net force is > perpendicular to the contours. Unsurprisingly, it's generally toward > the nearest point on the ring.
Someone should have told Niven --- doesn't his Ringworld spin? He could just have adjusted the thickness of the unobtainium layer to get whatever gravity he wanted.
> The central equilibrium point is unstable: a nudge off the axis will > send your test mass falling to the ring.
Even without flexibility. Oh, that's why it has to spin, of course --- to keep the flexible ring stable. Lars Mathiesen (U of Copenhagen CS Dep) <thorinn@...> (Humour NOT marked)


Anton Sherwood <bronto@...>