1. The problem statement, all variables and given/known data "a uniform steel rod of length 1.20 meters and mass 6.40 kg has attached to each end a small ball of mass 1.06 kg. The rod is constrained to rotate in a horizontal plane about a vertical axis through its midpoint. Find the moment of inertia of the ball-rod system." 2. Relevant equations I = (1/12)ML^2 I = MR^2 3. The attempt at a solution So, my friend was trying to help explain to me the solution to this, but I'm kind of stuck on it. See, what she did was: I(system) = I(ball) + I(rod) = MR^2 + (1/12)ML^2 = M(L/2)^2 + (1/12)M(L)^2 My question is why you can assume that the radius of the ball is apparently half of the length of the rod. That doesn't really seem like a logical conclusion to make.