My book states that impulse = Ft = mv_f - mv_i But it doesn't make sense to me because what if a Force of 5N is applied to an object for 10 seconds? Then I = Ft = 5N(10s) = 50 Ns. So if I apply 5N to an object with a friction force of 5N for 10s so that the object is traveling at a constant velocity, say 3m/s, for the period of time, then nothing has changed in the calculation because Ft = 5N(10s). However, if I make the same calculation using change of momentum, I get Impulse = mvf - mvi = m(3m/s) - m(3m/s) = 0. So why the different results if Ft = mv_f - mv_i ?