This formula is one of the foundations of physics. All of our equations are tied back into this, the amount of force applied is equal to the mass of the object multiplied by its acceleration. For these examples above we can see that they are flying in an arc, as soon as we shoot the cannon or let go of the ball we are throwing we are no longer applying a force to it. In fact the only forces on it should now be drag, caused by the air, and gravity pulling the ball down. the drag is  a source of friction, causing the ball or cannonball to slow down and gravity is what causes it to hit the ground again. So why does the air slow the projectile down? It is just gas right? Well while it might be  a gas it is also considered a fluid.