Whenever anything is set in motion, there must be some type of propulsive force that moves it. Propulsion is a key element of many activities, including athletic events, recreation, transportation, weapons, and space exploration. This article explains the basic principles involved in any propulsion system, differentiates the types of propulsion systems, and discusses some practical aspects of propulsion.

Basic Physics

Momentum/Impulse

To change an object's speed it is necessary to exert a net force on it for some duration of time. The product of the force and the length of time it is applied is called the impulse and is the same as the change of momentum. Any increase in speed is directly proportional to the impulse and inversely proportional to its mass:

A 1,000 kg (metric ton) car accelerating from 0–30 m/s (68 mph) in 6 seconds, thus requires an average force of 5,000 newtons. This is a force...