Okay, so what I'm confused with is. A 2D collision took place... It was an inelastic collision in the sense that energy was lost, but the objects did not stick together. So, I have two initial velocities and two final velocities When calculating the speed at the centre of mass, I know I have to multiply the velocities by the mass and divide it by the total mass... but do I only do that with the initial velocities? or both? Also, if I do that with both, do I only add the masses twice (in the divisor) or four times if i'm adding all the velocities? Also, after that step, to find the momentum, I know it's your mass x velocity... so the velocity that I'll plug in here will the velocity at the centre of mass... but for the actual masses itself, do I add the two initial masses? Let's say, the two masses are 20 and 50 kg. so do I multiply the velocity at the centre of gravity by 70? Please help! Thanks.