v denotes the vertical velocity of the ball that the boy is holding.
In this equation, u = horizontal velocity of the ball multiplied by the velocity of the car.
The diagram shown above is that which is seen by the observer walking down the sidewalk. The diagram is parabolic because the viewer can see both the vertical and horizontal components of the velocities on the graph.
The diagram above is that which is seen by the youngster who is sitting in the same car, which is travelling vertically up and down and is subjected to the influence of gravitation. In order for the automobile and youngster to catch up, the car must maintain a steady velocity.