What is the acceleration of a baseball when it momentarily stops at its maximum height after being tossed directly upward?

For a free-falling or an object thrown up, the only force acting upon it is the gravitational force. This gravitation pulls the object constantly towards the center of the Earth. As a result, during the whole motion, the force induces an acceleration with a constant value of 9.81 m/s². Therefore, the acceleration doesn't change anywhere and anytime in the whole duration of motion.