A projectile is any object projected into space (empty or not) by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more often refers to a weapon..
For details of the mathematics surrounding projectile trajectory, see equations of motion.
Arrows, darts, spears, and similar weapons are fired using pure mechanical force applied by another object; apart from throwing without tools, mechanisms include the catapult, slingshot, and bow.
Other weapons use the compression or expansion of gases as their motive force.
Blowguns and pneumatic rifles use compressed gases, while most other guns and firearms utilize expanding gases liberated by sudden chemical reactions. Light gas guns use a combination of these mechanisms.
Railguns utilize electromagnetic fields to provide a constant acceleration along the entire length of the device, greatly increasing the muzzle velocity.
Some projectiles provide propulsion during flight by means of a rocket engine or jet engine. In military terminology, a rocket is unguided, while a missile is guided. Note the two meanings of "rocket" (weapon and engine): an ICBM is a missile with rocket engines.
Ballistics analyze the projectile trajectory, the forces acting upon the projectile, and the impact that a projectile has on a target.
An explosion, whether or not by a weapon, causes the debris to act as multiple high velocity projectiles. An explosive weapon, or device may also be designed to produce many high velocity projectiles by the break-up of its casing, these are correctly termed fragments.
Many projectiles, e.g. shells, may carry an explosive charge or another chemical or biological substance. Aside from explosive payload, a projectile can be designed to cause special damage, e.g. fire (see also early thermal weapons), or poisoning (see also arrow poison).
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