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Isaac Newton proved the shell theorem saying that: 1. A spherically symmetric body affects external objects gravitationally as though all of its mass were concentrated at a point at its centre. 2. If the body is a spherically symmetric shell (i.e. a hollow ball), no gravitational force is exerted by the shell on any object inside, regardless of the object's location within the shell. 3. Inside a solid sphere of constant density the gravitational force varies linearly with distance from the centre, becoming zero at the centre of mass.