1. The problem statement, all variables and given/known data A small sphere of mass 2.0kg revolves at the end of a 1.2 m long string in a horizontal plane around a vertical axis. Determine its moment of inertia with respect to that axis. 2. Relevant equations Moment of inertia for sphere= (2/5)mr^2 3. The attempt at a solution I=(2/5)(2.0kg)(1.2m)^2 I=1.2 kgm^2 However, my book says to use the equation: I=mr^2. As a result they got I=2.9kgm^2. Why do they do this?? Isn't that the equation for Moment of inertia of a hollow cylinder?