Help a brainlet out bros.
Say we have a sphere centered at the origin of mass M and uniform density, with a cavity at (0,0,R/2). We can find the center of mass easily by taking the com of the sphere without the cavity, and the cavity, where we assume the cavity has negative mass. I know that taking negative mass is mathematically convenient and simplifies the problem but why can we assume a negative mass in the first place? Seems kinda nonsensical. Also how do i find the center of mass using a different method.