AQA GCSE Physics

Revision Notes

5.8.4 Factors Affecting Thinking Distance & Reaction Time

Factors Affecting Reaction Time

  • The thinking distance is defined as:

The distance travelled by a car from when a driver realises they need to brake to when they apply the brakes

  • The reaction distance is equal to:

Reaction Distance = Speed of the car × Driver’s reaction time

  • The main factor that affects the thinking distance is the car’s speed, however additional factors can affect the thinking distance
  • It is increased by:
    • Tiredness
    • Distractions (e.g. using a mobile phone)
    • Intoxication (i.e. consumption of alcohol or drugs)
  • Since these factors can affect the driver’s reaction time, they directly affect the thinking distance

Worked Example

The graph below shows how the thinking distance of a driver depends on the speed of the car.

WE Thinking distance question graph, downloadable IGCSE & GCSE Physics revision notes

(a) Describe the connection between thinking distance and speed.

(b) Some people drive when they are tired, despite warnings against doing so. Draw a new line on the graph to show how thinking distance varies with speed for a tired driver.

Part (a)

Step 1: Check if the line is straight and if it goes through the origin

    • The graph shows a straight line through the origin
    • Therefore, the thinking distance is directly proportional to the speed of the car

Part (b)

Step 1: Recall the factors which affect the thinking distance

    • Three additional factors affect the thinking distance, because they affect human reaction time:
      • Tiredness
      • Distractions
      • Intoxication
    • Hence, a tired driver’s reaction time is greater (i.e. it takes longer for them to react)

Step 2: Draw a line that shows greater thinking distance for the same speed

    • At the same speed, a tired driver’s thinking distance will be greater than a driver who is alert
    • This means a line should be drawn with a steeper gradient, as shown below:

WE Thinking distance solution graph, downloadable IGCSE & GCSE Physics revision notes

Author: Jonathan

Jonathan graduated with a first-class Master's degree in Theoretical Physics from Imperial College London. He has worked in education for more than a decade as a Maths and Physics Teacher, Tutor, Head of Physics, and most recently, as Assistant Headteacher. He is now an Educational Consultant and works with us to design and improve our Physics resources.
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