This is an excerpt from "Introduction to Mechanics" by Kleppner and Kolenkow: "The reason why gravitational force vanishes inside a spherical shell can be seen by a simple argument due to Newton. Consider the two small mass elements marked out by a conical surface with its apex at ##m##. The amount of mass in each element is proportional to its surface area. The area increases as (distance)2. However, the strength of the force varies as 1/(distance)2, where the distance is measured from the apex to the shell. Thus the forces of the two mass elements are equal and opposite, and cancel. We can pair up all the elements of the shell this way and so the total force on ##m## is zero." Are the cones similar because of the spherical symmetry of the shell?