1. The problem statement, all variables and given/known data There are four solid balls with their centres at the four corners of a square of side a. The mass of each sphere is m and radius is r. Find the moment of inertia of the system about one of the sides of the square. 2. Relevant equations 3. The attempt at a solution I= 2 (two balls) × [ 2/5 mr^2 (Icm) + ma^2 ] (just used parallel axis theorem) However, in the answer, I = 2 ( 2/5 mr^2 ) + 2×[ 2/5 mr^2 + ma^2 ] Why have they added 2 ( 2/5 mr^2 ) ??