I am having some difficulties understanding something here, it seems to me that the book at some point deny itself or I clearly do not get it. So it firstly states that: Inside the spherical shell the potential energy does not depend on radius from the center of the shell to the point of mass m and has the same value everywhere inside the shell. So the potential energy between the spherical shell and the point mass m is: U = -GMm/R, where M is the mass of the shell, and m - mass of a point mass inside the shell, R is the radius of the shell. So clearly this potential energy does not depend on where it is inside the shell and is always constant. Since the force is the negative gradient of the potential energy function, and U=const. the Force must be zero (everywhere inside the shell?) But now the book says: More generally, at any point in the interior of any spherical symmetric(radius R) mass distribution, at a distance r from its center, the gravitational force on a point mass m is the same as though we removed all the mass at points farther than r from the center and concentrated all the remaining mass at the center. So now it states that the force is not zero and can be calculated. First it said that the force on m at any point inside the shell must be zero. What am I missing here?