Changes in Momentum

You can use conservation of momentum to calculate velocities or masses

Momentum is conserved in a closed system. You can use this to help you calculate things like velocity or mass of objects in an events

Example question:

  1. Calculate the momentum of the pellet
  2. The momentum before the gun is fired is zero. This is equal to the total momentum after the collision
  3. The momentum of the gun is 1.5 x v
  4. Rearrange the equation to find the velocity of the gun. The minus sign shows the gun is travelling in the opposite direction to the bullet

Forces cause a change in momentum

  • You know that when a non-zero resultant force acts on a moving object it causes its velocity to change. This means that there is a change in momentum
  • You also know that F=ma and that a= change in velocity / change in time
  • So F=m x v-u/t which can also be written as: F = mchangeinv/changeint
  • The force causing the change is equal to the rate of change of momentum
  • A larger force means a faster change of momentum
  • Likewise, if someone’s momentum changes very quickly the forces on the body will be very large, and more likely to causes injury
  • This is why 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 if change in momentum, and so the smaller the force. Smaller forces means the injuries are likely to be less severe

Cars have many safety features, such as:

  • Crumple zones crumple on impact, increasing the time taken for the car to stop
  • Seat belts stretch slightly, increasing the time taken for the wearer to stop
  • Air bags inflate