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]
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.
> See http://www.ogre.nu/images/gringi.jpg
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.
> 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)