Stopping Distances

The stopping distance of a car is defined as:

The total distance travelled during the time it takes for a car to stop in response to some emergency

It can be written as an equation involving two distances:

Stopping distance = Thinking distance + Braking distance

Where:
- Thinking distance = the distance travelled in the time it takes the driver to react (reaction time) in metres (m)
- Braking distance = the distance travelled under the braking force in metres (m)
- Stopping distance = the sum of the thinking distance and braking distance, in metres (m)

For a given braking force, the greater the speed of the vehicle, the greater the stopping distance

Worked example

At a speed of 20 m/s, a particular vehicle had a stopping distance of 40 metres. The car travelled 14 metres whilst the driver was reacting to the incident in front of him. What was the braking distance?

A 54 m

B 34 m

C 26 m

D 6 m

Answer: C

Step 1: Identify the different variables

Stopping distance = 40 m
Thinking distance = 14 m

Step 2: Rearrange the formula for stopping distance

Stopping distance = Thinking distance + Braking distance

Braking distance = Stopping distance – Thinking distance

Step 3: Calculate and identify the correct braking distance

Braking distance = 40 – 14 = 26 metres
Therefore, the answer is C

Factors affecting Stopping Distance

There are various factors which can affect a vehicle's stopping distance. These are:
- Vehicle speed - the greater the speed, the greater the vehicle's braking distance will be (because the brakes will need to do more work to bring the vehicle to a stop)
- Vehicle mass - a heavy vehicle, such as a lorry, takes longer to stop
- Road conditions - wet or icy roads make it harder to decelerate
- Driver reaction time - a driver's thinking distance depends on their reaction time. Being tired, or intoxicated (i.e. alcohol, or drugs) can increase reaction time

Stopping Distances (Cambridge O Level Physics)

Revision Note