Im new to these forums and i'm looking forward to becoming a regular contributor. I'm attempting to prepare for physics in the fall and I have a textbook problem that i understand the process of solving but not understand the concept. Heres the question: What mass of a material w/ density p is required to make a hollow spherical shell having inner radius r1 and outer radius r2? I understand everything besides the inner and outer radius part. What is the textbook trying to say with that statement. How do you know to subtract them? So if you understood to subtract the radii can you please explain your thought process? Again thanks a alot.