Momentum and Collisions

What is Momentum?

Things which are moving have momentum. The faster they are moving and the greater their mass the more momentum they have. Try not to confuse momentum with kinetic energy. They are calculated in a different way and have different properties.

To calculate momentum we use the equation:

momentum = mass × velocity
               (kg m/s)                            (kg)                       (m/s)

You should remember from GCSE;

• Momentum has both magnitude and direction.

• When a force acts on a body that is moving, or able to move, a change in momentum occurs.

• Momentum is conserved in any collision/explosion provided no external forces act on the colliding/exploding bodies.

• When a collision happens the total amount of momentum must be the same before and after.

Force, change in momentum and time taken for the change are related by the equation:

force   =   change in momentum / time taken for the change
 (N)                         (kg m/s)                                      (s)

This means that the faster the momentum of an object changes, the bigger the force making it change. If we want to reduce the force which acts on objects in collisions we can do this by choosing certain materials and in the way we design the objects. A good example of this is in the design of cars.

Imagine a car hits a brick wall. If we can increase the time it takes for the driver to lose his / her momentum then the smaller the force slowing him / her down so the more likely they are not to get injured.

momentum change is fast  =  big force           momentum change is slow  =  smaller force

All the safety features below are designed to increase the time it takes for the momentum change to happen.

air bags	crumple zones	child seats	seat belts