# Impulse

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## Impulse

**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.

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.**