Part A Playing in the street, a child accidentally tosses a ball (mass m) with a speed of v=18 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.

Part B
When we combine the equation from Part A with the conservation of momentum equation, we can solve for both final speeds. This relationship will involve the masses of the ball and the car, but we can apply a simplifying assumption: the car is so massive compared with the ball that its speed will not change at all as a result of this collision. Translate this sentence into an equation, what is V′ equal to? Now, having made this assumption, it becomes possible to solve the equation from Part A for the final speed of the ball, what is it?