when we throw the ball vertically upward we give it a velocity in vertically upward direction which takes it upward. As the ball was moving with the car it had a horizontal velocity also which will take it in horizontal direction along with the car. so after a certain time it will cover a certain height and come down due to vertical velocity and acceleration due to gravity. and will also cover a certain distance in forwards direction due to initial horizontal velocity. so it will come back to the hand of the thrower only.