<Dd> F = d (m v) d t = m d v d t = m a, (\ displaystyle F = (\ frac (d (mv)) (dt)) = m (\ frac (dv) (dt)) = ma,) </Dd> <P> hence the net force is equal to the mass of the particle, times its acceleration . </P> <P> Example: A model airplane of mass 1 kg accelerates from rest to a velocity of 6 m / s due north in 2 s . The net force required to produce this acceleration is 3 newtons due north . The change in momentum is 6 kg ⋅ m / s . The rate of change of momentum is 3 (kg ⋅ m / s) / s = 3 N . </P> <P> In a closed system (one that does not exchange any matter with its surroundings and is not acted on by external forces) the total momentum is constant . This fact, known as the law of conservation of momentum, is implied by Newton's laws of motion . Suppose, for example, that two particles interact . Because of the third law, the forces between them are equal and opposite . If the particles are numbered 1 and 2, the second law states that F = dp / dt and F = dp / dt . Therefore, </P>

A quantity 15 m/s to the north is a measure of