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lark

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the force ON a sphere from another sphere, is the same as if the other sphere's mass were concentrated at its center.

So what is this force? It turns out that the force on a sphere FROM a point mass is the same as the force a sphere exerts ON a point mass - that is, the sphere is attracted to another point as if the sphere were a point itself! (from Newton's third law or whatever - action = reaction)

So the force between two spheres is the same as if the mass in the two spheres were concentrated at their centers.

In general if potential energy is 0 at infinity, the potential energy of object 1 from the gravitational field of object 2 is the same as the potential energy of object 2 in the grav. field of object 1 ...

laura