Stopping Distance (AQA GCSE Physics)

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Estimating Stopping Distances

  • For a given braking force, the speed of a vehicle determines the size of the stopping distance
  • The greater the speed of the vehicle, the larger the stopping distance
  • The image below shows how the stopping distance of a typical family car increases with increasing speed:

Highway Code, downloadable IGCSE & GCSE Physics revision notes

A vehicle's stopping distance increases with speed. At a speed of 20 mph the stopping distance is 12 m, whereas at 60 mph the stopping distance is 73 m (reproduced from the UK Highway Code)

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

Graphs Relating Speed to Stopping Distance

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

Graph showing how the velocity typically changes as a vehicle comes to an emergency stop

  • While the driver reacts (the time taken to press the brakes is called the reaction time), the vehicle continues moving at a constant velocity
    • The area underneath the graph during this time represents the thinking distance

  • As soon as the brakes are applied, the vehicle decelerates to a halt
    • The area underneath the graph during this time represents the braking distance

Worked example

While driving at a speed of 35 m/s, Stephen sees an obstacle in the road at time t = 0.The velocity-time graph below shows how the speed of the car changes as Stephen reacts and slams the brakes, bringing the car to a halt.WE stopping distance question graph, downloadable IGCSE & GCSE Physics revision notesDetermine

(a) The braking distance of the car.

(b) The driver's reaction time.

Part (a)

Step 1: Identify the section of the graph which represents the braking distance

    • The area under a velocity-time graph represents distance travelled
    • The braking distance of the car is the distance travelled under the braking force

    • This area of the graph is shaded below:

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

The braking distance of the car is the area shaded because the car decelerates once the brakes are applied

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

Part (b)

Step 1: Determine how long the driver takes before the brakes are applied

    • Between seeing the obstacle and applying the brakes, 1 second passes
    • This sequence of events is labelled on the graph below:

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

The driver's reaction time is the time between the moment they see the obstacle to the moment the brakes are applied

    • Therefore, the driver's reaction time is 1 s

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Ashika

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.