A baseball player is warming up and tosses a ball straight up into the air. The ball travels up until it reaches some maximum height, and then falls back down to the player/'s hand. If we neglect air resistance, then the magnitude of the acceleration of the ball is:

The only force acting on the ball, once it left the hand of the player, is the gravitational atraction of the earth. Once the ball reaches its maximum height, it starts a free fall movement under the action of earth's gravity.

Then the magnitude of the acceleration of the ball is the gravitational acceleration, g, approximatley 9.8 m/s^2