So there is a problem based upon the Center of mass and Moment of Inertia of continuous rigid bodies. So let's say we have a sphere of radius R and its mass is uniform throughout the system of particles. Now we cut out a smaller sphere of radius R/2 from the above sphere of radius R... Now my question is, Why can't we use another way of calculating the center of mass or moment of inertia of uniform bodies instead of using the concept of negative mass?