Changes in momentum

• We can use the fact that energy is conserved in a closed system to calculate things like the velocity or mass of objects in an event.
• When a non-zero resultant force acts on a moving object, it causes velocity to change. This means that there is a change in momentum.
• The formula for this is: force= change in momentum/ change in time
• The force causing the change is equal to the rate of change of momentum
• A larger force means a faster change of momentum
• If someone's momentum changes very quickly(like in a car crash) the forces on the body will b every large and are more likely to cause injury.
• Cars are designed to slow people down over a longer time when they have a crash- the longer it takes for a change in momentum, the smaller the rate of change of momentum, and so the smaller the force. Smaller forces mean the injuries are likely to be less severe.
• Cars have many safety features, including:

  Crumple zones- crumple on impact increasing time taken for the car to stop.

  Seat belts stretch slightly, increasing time taken for the wearer to stop.

  Air bags inflate before you hit the dashboard of a car. The compressing air inside it slows you down more gradually than if you had just hit the hard dashboard.
• Bike helmets contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash. This reduces the impact on your brain.
• Crash mats and cushioned playground flooring increase the time taken for you to stop if you fall on them. This is because hey are made from soft, compressible materials.