A projectile is a body which is propelled (or thrown) with some initial velocity, and then allowed to be acted upon by the forces of gravity and possible drag. The maximum upward distance h reached by the projectile is called the height, the horizontal distance traveled x the range (or sometimes distance), and the path of the object is called its trajectory. If a body is allowed to free-fall under gravity and is acted upon by the drag of air resistance, it reaches a maximum downward velocity known as the terminal velocity. The study of the motion of projectiles is called ballistics.