Braking & Reaction Times (OCR A Level Physics)

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Katie M

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Katie M

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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, downloadable IGCSE & GCSE Physics revision notes

Estimate of stopping distances for various speeds

  • The image illustrates the following important principles:
    • The thinking distance increases proportionally with speed (i.e. if speed doubles, the thinking distance doubles)
    • The braking distance increases at an even faster rate with speed

  • For a typical family car, these speeds and stopping distances are summarised in the table below:

Table of Stopping Distances for a Family Car

Estimating stopping distance table, downloadable IGCSE & GCSE Physics revision notes

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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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.