Applications of Projectile Motion?

The stay airborn because sombody exerted a force that compelled them to accelerate. The force has two components, the vertical component which lofts the ball into the air and the horizontal component which moves it forward. The two motions are completely independent of each other. The reason that the ball eventually comes back down to Earth is because gravity exerts a downward force that slows the upward trajectory of the ball and then cause it to accelerate downward back to Earth.