Why does the velocity change when the boy hits the ball

Freefall is a special case of motion with constant acceleration because acceleration due to gravity is always constant and downward. This is true even when an object is thrown upward or has zero velocity.

When a ball is thrown up in the air, its velocity is initially upward. Since gravity pulls the object toward the earth with a constant acceleration g, the magnitude of velocity decreases as the ball approaches maximum height. At the highest point in its route, the ball has zero velocity, and the magnitude of velocity increases again as the ball falls back toward the earth.

where is the circuit diagram ?

close lines strong magnet less close weaker