1. The problem statement, all variables and given/known data EDIT: DO NOT READ. I SOLVED IT. The collision in this problem acts at point C and acts a purely horizontal force in the inertial frame of reference. 2. Relevant equations 3. The attempt at a solution I understand the maths that is at work here, when we resolve the force into the new frame of reference, x1 and y1, we can find the moments and develop the angular momentum equations. However, what is bothering me is that there is a moment about the Z (or Z1, it's the same) axis despite the collision force acting through line of the centre of gravity? This is deeply annoying to me, because I'd think this physically impossible. So, I'm not sure where the hole in my physical understanding of the problem is.