An impulse is the integral of a force (or thrust) over time. If the force is constant, the impulse is simply the force multiplied by the time. By Newton's second law (see Newton's laws of motion), the impulse (a vector quantity) equals the change of momentum produced by the force. It is a useful concept when a large and variable force acts for a short time, as in an impact. It is measured in newton seconds (N.s).