OCR A Level Physics

Revision Notes

3.2.4 Braking & Reaction Times

Braking & Reaction Times

  • The stopping distance of a car is the distance it travels in the time it takes to stop in response to an emergency
  • The stopping distance consists of two parts:
    • The thinking distance
    • The braking distance
  • Therefore, the stopping distance is equal to:

Stopping distance = Thinking distance + Braking distance

  • The graph below shows how the velocity of a car will typically change during an emergency stop

stopping-distance-graph, IGCSE & GCSE Physics revision notes

A velocity-time graph for a stopping car showing thinking and braking distance

  • The stopping distance increases considerably with the speed of a car
    • This is mostly because a car will travel further whilst braking before coming to rest

Highway-Code, IGCSE & GCSE Physics revision notesEstimate of stopping distances for various speeds


Thinking Distance

  • Thinking distance is defined as:

The distance travelled by the vehicle from when the driver sees a problem and the brakes are applied

  • The thinking distance is proportional to the initial speed, u, of the vehicle
  • The thinking distance can be calculated from:

Thinking distance = Initial speed × Reaction time

  • Where:
    • Reaction time =  the time taken by the driver to respond to the problem
  • Factors that affect thinking distance are:
    • The initial speed of the vehicle
    • Intoxication ie. consumption of alcohol or drugs
    • Distractions ie. using a mobile phone
    • Tiredness (since reaction times increase when tired)

Braking Distance

  • Braking distance is defined as:

The distance travelled by the vehicle after the driver has applied the brake

  • The braking distance of the vehicle is proportional to the square of the initial speed of the car, u2
  • This is because all of the vehicle’s kinetic energy (½ mv2) must be dissipated by the brakes in order to come to a stop
  • The work done by the brakes is given by:

Work Done = Braking Force × Braking Distance = ½ mv2

  • Factors that affect braking distance are:
    • The initial speed of the vehicle
    • Mass of the vehicle
    • Poor road conditions eg. icy, wet
    • Car conditions eg. worn brakes

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