A tennis ball with a speed of 15.7 m/s is moving perpendicular to a wall. After striking the wall, the ball rebounds in the oppo
site direction with a speed of 14.444 m/s. If the ball is in contact with the wall for 0.0099 s, what is the average acceleration of the ball while it is in contact with the wall? Take "toward the wall" to be the positive direction. Answer in units of m/s 2 .