AQA GCSE Physics

Revision Notes

5.9.5 Momentum & Safety

Higher Tier Only

Momentum & Safety

  • Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs
    • The contact time is the time in which the person is in contact with what they have collided with
  • Therefore, safety features are created to reduce the impact of a force, such as in:
    • Vehicles
    • Playgrounds
    • Bicycle helmets
    • Gymnasium crashmats


  • Vehicle safety features are designed to absorb energy upon an impact by changing shape
  • The main vehicle safety features are crumple zonesseat belts and airbags
    • For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place
    • This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest
    • The increased time reduces the force and risk of injury on a passenger
  • The usefulness of safety equipment depends on two main factors: mass and velocity
  • If the impact is from a large mass, for example, a truck travelling very fast and colliding with a wall, the momentum will be very large
    • The change in momentum (ie. from a high speed to rest) will also be very large
    • This means that a very long contact time is needed to reduce the force of impact

Car Safety Features, downloadable AS & A Level Physics revision notes

The seat belt, airbag and crumple zones help reduce the risk of injury on a passenger

  • Seat belts
    • These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
    • They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision
  • Airbags
    • These are deployed at the front on the dashboard and steering wheel when a collision occurs
    • They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
  • Crumple zones
    • These are designed into the exterior of vehicles
    • They are at the front and back and are designed to crush or crumple in a controlled way in a collision
    • This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
    • The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

Crash Mats

  • Crash mats used in gymnasiums help reduce the risk of injury for falls in gymnastics and climbing
    • They are thick and soft to offer shock absorption of the force created by the person landing on the mat
  • When a person lands on a crash mat with a large force, for example after jumping, the soft landing means their body is in contact with the mat for a longer period of time than if it was otherwise not there
  • This increases the contact time over which their momentum is reduced creating a smaller impact force and a lower chance of injury

A bouldering mat is a type of crash mat used to reduce the chance of injury in falls whilst climbing

  • In a similar way, playgrounds utilise cushioned surfaces as children will often fall onto these with a large force
    • The cushioned surface reduces the risk of a severe injury by increasing their contact time with the ground
  • Meanwhile, a child in a gymnasium can use a thinner crash mat than an adult due to having a lower mass
  • This is the same for activities where a person/adult will fall with a low velocity such as falling from lower heights
    • Therefore, thin crash mats are suitable for low-impact activities
  • Safety features are intended to reduce the chance of serious injury but do not completely prevent it in all cases

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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