Ballistics (from Greek βάλλειν ballein, "to throw") is the science of mechanics that deals with the launching, flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body with momentum which is free to move, subject to forces, such as the pressure of gases in a gun or a propulsive nozzle, by rifling in a barrel, by gravity, or by air drag.A ballistic missile is a missile only guided during the relatively brief initial powered phase of flight, whose trajectory is subsequently governed by the laws of classical mechanics, in contrast (for example) to a cruise missile which is aerodynamically guided in powered flight.