Calculating Moment of Inertia. For a discrete collection of point masses we found ∑m;r For a continuous solid object we have to add up the mr2 contribution for every infinitesimal mass element dm dm Ne have to do an integral to find l rdm Physics 121: Lecture 18, Pg 11Physics 121: Lecture 18, Pg 11 Calculating Moment of Inertia... For a discrete collection of point masses we found: For a continuous solid object we have to add up the mr2 contribution for every infinitesimal mass element dm. We have to do an integral to find I : I = = m r i i i N 2 1 r dm I =  r dm 2