A collision between two or more bodies occurs when the surfaces of the bodies come into contact with each other even as the bodies are moving relative to one another. The collision typically results in a large amount of force (including normal force and friction) exerted over a short time period. The force exerted is more conveniently measured in terms of the impulse, which is the integral of force over the time interval of the collision. The result of the collision is usually a change in the linear momentum of the individual colliding bodies (though not of the system). If the collision is an inelastic collision, there is also some loss of kinetic energy which may be released as heat, light, or sound.