The higher the impulse of a particular motor, the larger and faster and higher it will cause the rocket to travel. Impulse is a term that describes the relative strength of a rocket motor. The impulse of a given engine is found by multiplying the average force that the motor will push with by the number of seconds that the motor will push. Model rocket motors are usually measured using the metric system, where the units of force are "newtons". If a motor pushed with a force of 10 newtons for 3 seconds, this motor would have an impulse of 30 newton-seconds (usually written 30 N-s). This is the same impulse as a motor which pushes with a force of 3 newtons for 10 seconds. When choosing between two motors with the same impulse, the decision to use a shorter duration with higher force or longer duration with lower force, is made based on the amount of drag and the mass of the rocket and other factors, but these factors are usually much less important than selecting the impulse of the engine. In the United States, model rocket motors use a letter desigtation followed by a pair of numbers, such as "B-6-4". The letter describes the approximate impulse of the motor. "A" motors have around 2.5 N-s of impulse, "B"s have twice as much impulse as "A"s, "C"s have twice as much impulse as "B"s etc.