AQA A Level Physics

Revision Notes

4.5.5 Collisions

Elastic & Inelastic Collisions

  • In both collisions and explosions, momentum is always conserved
  • However, kinetic energy might not always be
  • A collision (or explosion) is either:
    • Elastic – if the kinetic energy is conserved
    • Inelastic – if the kinetic energy is not conserved
  • Collisions are when objects striking against each other
    • Elastic collisions are commonly those where objects colliding do not stick together and then move in opposite directions
    • Inelastic collision are commonly those where objects collide and stick together after the collision

Elastic & Inelastic Collisions, downloadable AS & A Level Physics revision notes

Elastic collisions are where two objects move in opposite directions. Inelastic collisions are where two objects stick together

  • An explosion is commonly to do with recoil
    • For example, a gun recoiling after shooting a bullet or an unstable nucleus emitting an alpha particle and a daughter nucleus
  • To find out whether a collision is elastic or inelastic, compare the kinetic energy before and after the collision
  • The equation for kinetic energy is:

Kinetic energy equation, downloadable AS & A Level Physics revision notes

Worked Example

Trolley A of mass 0.80 kg collides head-on with stationary trolley B at speed 3.0 m s–1. Trolley B has twice the mass of trolley A.

The trolleys stick together and travel at a velocity of 1.0 m s–1. Determine whether this is an elastic or inelastic collision.

Collisions Worked Example Answer (1), downloadable AS & A Level Physics revision notesCollisions Worked Example Answer (2), downloadable AS & A Level Physics revision notes

Exam Tip

If an object is stationary or at rest, it’s velocity equals 0, therefore, the momentum and kinetic energy are also equal to 0.

When a collision occurs in which two objects are stuck together, treat the final object as a single object with a mass equal to the sum of the two individual objects.

Momentum Conservation Issues

  • 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 vehicle or the passenger is in contact with what it has collided with
  • Vehicles have safety features such as crumple zones, seat belts and airbags to account for this
    • 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

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

Designing Safety Features

  • Vehicle safety features are designed to absorb energy upon an impact by changing shape
  • 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


  • The effect of the increase in time and force can be shown on a force-time graph
    • For the same change in momentum, which depends on the mass and speed of a vehicle, the increase in contact time will result in a decrease in the maximum force exerted on the vehicle and passenger
    • This is demonstrated by a lower peak and wider base on a force-time graph

Seat Belt Force -Time Graphs, downloadable AS & A Level Physics revision notes

The increase in contact time Δt decreases the force for the same impulse

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