Two friends, Al and Jo, have a combined mass of 168 kg. At an ice skating rink they stand close together on skates, at rest and facing each other, with a compressed spring between them. The spring is kept from pushing them apart because they are holding each other. When they release their arms, Al moves off in one direction at a speed of 0.81 m/s, while Jo moves off in the opposite direction at a speed of 1.5 m/s. Assuming that friction is negligible, find Al's mass. I am stuck on this problem. I think I have to use this equation: m1*v1 + m2*v2=(m1+m2)vf. But, I don't know how I would use it to solve this problem. Can someone please help me figure out how to solve this problem?