Changing momentum From Newton's Second Law and the definition of force: (mv = final momentum, mu = initial momentum) So, Ft = mv - mu But, mv - mu = change in momentum We call change in momentum impulse. Impulse = Change in momentum = Ft Units: Ns or Kgm/s To achieve any particular change in momentum, you can either have a large force multiplied by a small time or a small force multiplied by a large time. This explains how crumple zones work! Volvo cars love collisions so much that they have crumple zones which make the collisions last longer. A longer time means that you can achieve the same change in momentum during a collision by applying a smaller force. And it is the force that hurts the passengers. Force-time graphs We can plot graphs of the force during a collision against time. We can find the impulse, or change in the momentum, by calculating the area under the force-time graph. Question:. A car of mass 1000 kg is travelling at 40 m/s when it collides with a wall. The passenger can only survive when the maximum force on the car is 20 000 N. /**/