1. The problem statement, all variables and given/known data If the truck was not stationary but was moving horizontally towards the ball at 25 ms-1, what would be the speed of the ball, relative to the ground, after the collision (in ms-1)? Mass of ball = 0.2 kg Mass of truck = 20 tonnes Initial velocity of ball = 20 ms-1 (to the right) Initial velocity of the truck = 25 ms-1 (to the left) This is also a perfectly elastic collision 2. Relevant equations P = mv 3. The attempt at a solution So what I did first was equated the sum of the initial momentum's to the sum of the final momentum's to try and figure out what the final velocity of the ball was however it just gave me 20 which is incorrect. so... m1v1initial + m2v2initial = m1v1final + m2v2final I found that because we can cancel the final and initial momentum's of the truck due to its change of velocity being negligible I can equate the initial and final momentum's of the ball. However when I do this it gives me a velocity of 20 which is still incorrect. Thank you in advance.