# Understanding Newtonian Gravitation

1. Apr 9, 2013

### Quantumsatire

So for a point mass in an enclosed shell, the net force of gravity is zero (similar to electricity in a Faraday cage I presume). However, what happens when that point mass is placed in side the ring of mass m and uniform density. Say the outer shell has radius r and inner shell has radius x, so the region r-x is a massed shell/ring. How would you find the net force of gravity?

2. Apr 9, 2013

### BobG

If the outer shell alone gives you a net force of zero, then it can be ignored.

You only consider the inner shell and calculate your force in the normal way.

3. Apr 10, 2013

### Puneeth423

Net gravitational force on point mass by ring will be always zero. Consider the entire mass of ring as small point equal masses. For every point mass on ring, there will be another point mass symmetrically opposite on the ring. Net force by two opposite masses on the point mass inside cancel out. Thus, net gravitational force is zero

4. Apr 10, 2013

### sophiecentaur

Do you mean spherical shell or an annulus? They are not the same and the shell theorem only applies to a spherical shell.