1. The problem statement, all variables and given/known data A self-gravitating ball of gas suddenly experiences a rapid increase in its pressure to a new value P'. Taking average density of the star to be ρ and radius R, and ignoring gravity altogether, estimate the time scale of expansion of the star. 2. Relevant equations Force= Pressure/Surface area= mass x Acceleration 3. The attempt at a solution Considering an outer shell of thickness δr and radius r: Outward force on shell = (P(r) - P(r + δr)) 4πr2 My question is really why is the P(r + δr) the minus of it? I'm comfortable doing the rest of the question by just using equation mass of shell x acceleration and equating them and using dimensional analysis on that . But I'm unsure why it's a minus there, i'm fairly certain that's right after speaking classmates but I don't understand why. Thanks in advance.