The newton-second (N · s) is the standard unit of impulse. Reduced to base units in the International System of Units ( SI ), a newton-second is the equivalent of a kilogram-meter per second (kg · m/s or kg · m · s -1 ). In terms of literal units, impulse is equivalent to momentum, although there is a conceptual difference between the two phenomena.
As an example of impulse and its relationship with momentum, consider a rocket ship coasting through space at a speed of 10,000 (10 4 ) m/s. Suppose the ship's forward thrusters are fired for a period of 10 s, and they produce a total force of 100,000 (10 5 ) N. The impulse I generated is equal to the product of the force F and the time t :
I = Ft
= 10 5 kg · m/s 2 · x 10 s = 10 6 kg · m/s
A rule of Newtonian physics states that the impulse imparted to an object is equal to the change in momentum for that object, provided no other forces or effects are involved. Therefore, by firing the rockets so an impulse of 10 6 kg · m/s is produced in the forward direction, the forward momentum of the vessel is increased by 10 6 kg · m/s. If the retro rockets are fired so the impulse is contrary to the direction of the ship's motion, then the forward momentum of the ship will be reduced by 10 6 kg · m/s.
The equivalence of impulse and change in momentum holds regardless of the mass of the object to which the impulse is applied.