GCSE Physics (AQA)

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Car Safety Features

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Forces Momentum

Car Safety Features

3:51 Force as Change of Momentum
Spec 4.5.7.3
Previous Momentum & Safety Next Explosions & Recoil
  • During a crash, a large force causes a car to lose all momentum.
  • Before the crash, both cars had momentum, calculated as the product of their mass and velocity, approximately 24,000 kg m/s for each car.
  • The crash causes the cars to stop, resulting in a momentum change from 24,000 to zero.
  • This change in momentum is due to the contact force between the two cars.
  • Force causes a change in momentum, and its magnitude determines the rate of change of momentum, expressed as F = delta P / delta T.
  • The change in momentum is large and occurs in a short time, resulting in a large reaction force.
  • This large force is dangerous for passengers, potentially causing harm.
  • Cars have safety features to reduce the effects of this force on passengers.
  • The momentum lost in a crash cannot be changed, but the time taken for this momentum to be lost can be increased, reducing the force to a safer value.
  • Crumple zones increase the time it takes for a car to stop by collapsing on themselves during a crash, reducing the average force on the car.
  • Seat belts slowly bring passengers to a stop, preventing them from hitting the dashboard or seat in front with a large force.
  • Seat belts stretch when passengers push against them, increasing the time to stop and reducing the force.
  • Airbags cushion passengers as they hit the dashboard, inflating automatically during a crash to gradually stop the passenger's head, increasing stopping time and decreasing force.
  • In exams, students may be asked to provide examples of car safety features or explain how they reduce impact force.

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