Re: Gravity Inside A Cylinder

From: JM Albuquerque (jm.aREMOV.E_at_sapo.pt)
Date: 08/07/04 

Date: Sat, 7 Aug 2004 22:22:13 +0100

"Jaxtraw" <jaxtraw@nospamnobigfoot.com> wrote:

> "JM Albuquerque" <jm.aREMOV.E@sapo.pt> wrote:
> > Once upon a time I had a great discussion about that subject and I'm
> > going to tell you what I've learned so far.
> > The gravity field in a given point is the volume integral of all
> > elementary
> > masses, applying the usual Newton's formula F=GMm/d^2, being M the
> > mass of your test mass object and "m" all the elementary mass at
> > distance "d".
> > Concluding, for a spherical object (like the earth) the gravity at the
> > center of the earth is null. For a donut shaped massive object the
> > gravity at the center of donut's mass is null.
> > That's it: "The gravity at the center of mass is null"
> > Any deviation from the center of mass gives some gravitic force
> > calculated by means of the above said integral.
> > In the case of your cylinder the gravity is null at point O.
>
> Surely, it's also true that within a spherical shell there is no
> gravitational force felt by an object *anywhere* inside it?

Not anywhere inside.
Just at the center of mass (more below).
Also notice that a thin shell produces small gravity, therefore the gravitic
force is always small. The amount of mass is fundamental.
Your cylinder produces almost nothing and probably cannot even be
measured, unless you got a special material with a density 1000 times
more what we ever saw. Moreover, you have a balanced situation at the
axis of the cylinder (if homogeneous). Only near the wall you will got
something to measure.

> > Objects on the axis will be balanced and don't fall to the walls.
>
> What about off axis objects? That was the crux of my question really,
> whether they would feel a net gravitational pull towards the walls or not.

Out of the axis a small unbalance results in a net force.
It will be like a stopped object balanced between the Earth and the Moon
that doesn't know if it is going to fall to the Earth or to the Moon.
Any unbalance will cause the object to fall, but very slowly, since it
starts almost balanced.

But this is all speculation, since nobody knows what gravity really is.

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