When a pop fly is hit into the air, it is at its greatest point of gravitational potential energy right before it begins to fall down. At this point, the energy begins to convert to kinetic energy. The ball falls at a rate of 9.8 m/s, because that is the speed of acceleration, stated by the Law of Universal Gravitation. The ball falls to the ground because of gravity; the earth is pulling the ball to it. The baseball is very dense in mass, so the gravitational force is greater than, for example a balloon, because the more mass an object has, the greater its gravitational force.