1. The problem statement, all variables and given/known data The problem is pretty simple, however I don't understand which value to use after using quad formula to solve. See below. V1(initial) = 2.5 m/s V2(Initial) = -5.9 m/s A pool ball moving with a speed of 2.5 m/s makes an elastic head-on collision with an identical ball traveling in the opposite direction with a speed of 5.9 m/s. Find the velocities of the balls after the collision. 2. Relevant equations ΣP(initial) = ΣP(final) M1V1 + M2V2 = M1V1' + M2V2' M1 = M2 2.5 + (-5.9) = V1' + V2' V2' = -V1' - 3.4 Elastic collision: KE(initial) = KE(final) (Mass cancels out) V12+V22 = V1'2+V2'2 3. The attempt at a solution From above, I got: 0 = 2V1'2 - 6.8V1' - 29.5 by substituting "V2' = -V1' - 3.4" for V2' After applying quadratic equation I got: V1' = -2.5 m/s, 5.9 m/s My problem is I do not know which one is the correct answer. My answer key says V1' is equal to -2.5 m/s, but I have no idea why. Any help is appreciated!