A1000kg car is moving at 10m/s. a 400n force is on the car for 5 seconds. what is the change in momentum of the car.

From Newton's second law of motion,
F = m*a
   = m * (change in velocity)/time [As a = (change in velocity)/time]

From the question, we know
F = 400 N, time = 5 s

Therefore,
400 = 1000 * (change in velocity)/5
i.e. change in velocity = 2 m/s

Momentum = m * velocity
Therefore,
Change in momentum = (change in mass) * (change in velocity)
                                    = 1 * (change in velocity) [As m remains the same]
                                    = 1 * 2
                                    = 2 kgm/s