Forces - Momentum

When a resultant force acts on an object, it will cause a change in momentum that acts in the direction of the force. The longer this force is applied, the greater will be the change in momentum.

An example of this in everyday life is the way that you catch things, particularly with hard objects. As you catch them, you move your hands in the direction of the object's momentum. This means that the change in momentum takes place over a longer time period, so less force is required to stop the object in flight and it hurts your hands less. Accelerating or decelerating is another example - a smaller force acting over a longer time can produce the same momentum change.

The idea of momentum is also used in vehicle safety. Seat belts stretch and 'give' slightly during a collision in order to increase the length of time taken to stop you moving. This means that a lower force is exerted on your body resulting in less damage. Crumple zones of vehicles achieve the same, as do airbags which compress as your body hits them. Outside of the vehicle, guardrails are also designed to be flexible for the same reason, less force is required to stop a vehicle than if they were rigid so there should be less damage to the occupants.

If there are no external forces acting on a system, the total momentum remains the same during a collision. This is known as the conservation of momentum and a favorite of the examiners is to describe two objects colliding, sticking together and then asking you to work out the velocity after the collision. At first, answering such questions may seem difficult but if you keep in mind the conservation of momentum and the equation for momentum, they are not too bad, as you will hopefully realize by the end of this quiz.