Projectile motion with the effect of drag force It is certain that Galileo discovered the parabolic trajectory no later than 1608 and proved it mathematically early in 1609, although he did not mention it in print until 30 years later. A projectile is any particle that is given an initial velocity and then follows a path determined entirely by the effects of gravitational acceleration and air resistance. A thrown football, a package dropped from an airplane, and a bullet shot from a rifle are all projectiles. The path followed by a projectile is called its trajectory. The key to analyzing projectile motion is that we can treat the x- and y-coordinates separately. The x-component of acceleration is zero, and the y-component is constant and equal to (By definition, g is always positive; with our choice of coordinate directions, is negative.) So we can analyze projectile motion as a combination of horizontal motion with constant velocity and vertical motion with constant acce...