Factors Affecting Stopping Distance (Edexcel GCSE Physics)

• 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!

• 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

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:

Step 2: Calculate the area under the graph during the car's deceleration

• • The area is a triangle, so the braking distance is given by:

Braking distance = Area = ½ × base × height

Braking distance = ½ × (4.5 – 1) × 35 = 61.3 m