I've been killing myself over this problem because I cannot figure it out. Can someone please help me? A tennis ball with a velocity of 40.0 m/s to the right is thrown perpendicularly at a wall. After striking the wall, the ball rebounds in the opposite direction with a velocity of -8 m/s to the left. If the ball is in contact with the wall for 0.014 s, what is the average acceleration of the ball while it is in contact with the wall?