1. Why is 2(pie)rh multiplied by the dr (change in r) equal to the change in volume? This is in the case of deriving the moment of inertia of a cylinder; the equation is (change in mass)=2(pie)(radius)(height)(change in radius)(density). 2. Relevant equations 3. The attempt at a solution I don't quite understand how 2(pie)(radius)(volume)(change in radius) equals change in volume? Is there a proof for this? I can imagine this working mathematically but not conceptually.2(pie)(radius)(height) is just the surface area for each layer, but how does the change in radius come in? I know it has something to do with accounting for the thickness of each hollow cylinder? All replies are much appreciated. thanks.