Question: what is the acceleration of a ball when it reaches its highest point?
Answer: At the very top of the ball's motion, it has a velocity of zero. At the very top of it's motion, the ball has an acceleration of -9.8 m/s^2. Nothing happens to the direction of the ball's motion and on the way up the speed decreases. At the very top the speed is 0 and the acceleration is equal to the force of gravity.