This is because if they are on ice, there is very little friction and the only external forces on the hockey players are gravity and the normal force, which lead to a net zero force. (I assume this is...This is because if they are on ice, there is very little friction and the only external forces on the hockey players are gravity and the normal force, which lead to a net zero force. (I assume this is why we are using hockey players instead of football/soccer/rugby/etc. Also, if the system is isolated, the center of mass velocity should be the same before and after the collision.
