1. The problem statement, all variables and given/known data Playing in the street, a child accidentally tosses a ball (mass m) with a speed of v=23 m/s toward the front of a car (mass M) that is moving directly toward him with a speed of V=20 m/s . Treat this collision as a 1-dimensional elastic collision. After the collision, the ball is moving with speed v′ back toward the child and the car is moving with speed V′ in its original direction. My options are: A) v + V = v' - V' B) v - V = -v' + V' C) v - V = v' -V' D) v + V = -v' + V' 2. Relevant equations Conservation of Energy Conversation of Momentum 3. The attempt at a solution I took the movement of the child's ball as the positive direction and the car's direction as the negative. So this would give me v - V = -v'-V' but that is not an option. Can someone point me in the proper direction? EDIT: For all of those coming for a quick answer for MasteringPhysics, the answer is v + V = v' -V'