Suppose I have the system on the attached file, where two slender rods collide. I want to calculate the angular momentum of the system before collision relative to the center of mass of the two rods. I'm however not sure how to do this. Definately the formula L=Iw doesn't apply, since I don't know from which point I should measure the angular velocity. Equivalently I don't know where what R to use if I use the formula: L = mvR. It seems intuitive if it was the center of mass, but why it is, I have no idea. Who can explain what R to use and why?