Stopping Distances (Cambridge (CIE) O Level Physics): Revision Note

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

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