1. The problem statement, all variables and given/known data A man on a boat throws behind him a rock of mass m1=2kg with a velocity of v1=18m/s. The mass of a man with a boat is m_2=90kg. What velocty v_2 will the boat go with? 2. Relevant equations v2 = (m1*v1)/m2 3. The attempt at a solution So I was upsent on the lesson where we were speaking about this and can't figure out where does the formula I wrote above come from. Why is it certain that m2v2=m1v1? Of course I know I can just plug the numbers inside and bam, I've got my answer but I'd rather know why does it work. Could you help?