[kml_flashembed movie=”http://www.helpmyphysics.co.uk/impulse.swf” height=”450″ width=”550″ /kml]

When a car is in collision with a wall which brings it to a stop its momentum must be reduced to zero.

The change of momentum of the car is equal to the impulse it receives.

Impulse = change of momentum.

Ft = change of momentum

Now this change of momentum can come about with a LARGE FORCE acting over a small time or a small force acting over a LONG TIME.

The above movie shows the force time graph of two collisions of a car with a wall and then a collapsible wall.

Assuming the car is travelling at the same speed before each collision the change of momentum will be the same in each case.

You can calculate the change of momentum from the area of the force time graph. Note each graph has the same area but the time to change the momentum is different.

You will notice that in the collision with the collapsible barrier the time for the change in momentum is longer. This means that to keep the same area the peak force will be lower.

So to survive collisions it is important to increase the time you take to come to a stop. That is why car safety needs airbags, seat belts and crushable car fronts.