Factors Affecting Stopping Distance (Edexcel GCSE Physics: Combined Science)

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Factors Affecting Braking Distance

  • The braking distance is defined as:

The distance travelled by a car under the braking force - i.e. whilst it is slowing down

  • The main factor affecting the braking distance of a car is its speed
    • The greater the speed, the greater the braking distance will be

  • There are additional factors which affect the braking distance, such as:
    • Vehicle condition - e.g. worn tyres or poor brakes
    • Road condition - wet or icy roads make it harder to decelerate
    • Vehicle mass - a heavy vehicle, such as a lorry, takes longer to stop

  • The smoother the road conditions, for example when they are wet and icy, the less friction there is between the tyres and the road surface so there would be a greater braking distance
  • The braking distance is the ratio of the kinetic energy of the car and the braking force
    • This is because the work done in bringing a car to rest is the transfer of all its kinetic energy into other forms (thermal, sound)

  • The kinetic energy is equal to

KE = ½mv2

  • This means the braking distance is proportional to the velocity squared
    • If the velocity doubles, the braking distance increases by (2)2 , 4 times!

Distance proportional to velocity squared, downloadable IGCSE & GCSE Physics revision notes

Factors Affecting Thinking Distance

  • 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.
 

Answer:

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 that affect the thinking distance (as they affect human reaction time) are:
    • 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 an alert driver
  • 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

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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.