I'm trying to plug in negative mass into conservation of momentum and energy equations. I want to see what would happen if a -1kg ball traveling at 1m/s smacks into a 2kg ball in an elastic collision. If the second ball is 1kg, then it remains stationary while the negative ball continues to travel at 1m/s, like they didn't even interact. If its 2kg, I get 2 answers for the speed of the positive ball, zero and negative two. How do I know which is correct?