Braking & Reaction Times
- The stopping distance of a car is the distance it travels in the time it takes to stop in response to an emergency
- The stopping distance consists of two parts:
- The thinking distance
- The braking distance
- Therefore, the stopping distance is equal to:
Stopping distance = Thinking distance + Braking distance
- The graph below shows how the velocity of a car will typically change during an emergency stop
A velocity-time graph for a stopping car showing thinking and braking distance
- The stopping distance increases considerably with the speed of a car
- This is mostly because a car will travel further whilst braking before coming to rest
Estimate of stopping distances for various speeds
- The image illustrates the following important principles:
- The thinking distance increases proportionally with speed (i.e. if speed doubles, the thinking distance doubles)
- The braking distance increases at an even faster rate with speed
- For a typical family car, these speeds and stopping distances are summarised in the table below:
Table of Stopping Distances for a Family Car
Thinking Distance
- Thinking distance is defined as:
The distance travelled by the vehicle from when the driver sees a problem and the brakes are applied
- The thinking distance is proportional to the initial speed, u, of the vehicle
- The thinking distance can be calculated from:
Thinking distance = Initial speed × Reaction time
- Where:
- Reaction time = the time taken by the driver to respond to the problem
- Factors that affect thinking distance are:
- The initial speed of the vehicle
- Intoxication ie. consumption of alcohol or drugs
- Distractions ie. using a mobile phone
- Tiredness (since reaction times increase when tired)
Braking Distance
- Braking distance is defined as:
The distance travelled by the vehicle after the driver has applied the brake
- The braking distance of the vehicle is proportional to the square of the initial speed of the car, u2
- This is because all of the vehicle’s kinetic energy (½ mv2) must be dissipated by the brakes in order to come to a stop
- The work done by the brakes is given by:
Work Done = Braking Force × Braking Distance = ½ mv2
- Factors that affect braking distance are:
- The initial speed of the vehicle
- Mass of the vehicle
- Poor road conditions eg. icy, wet
- Car conditions eg. worn brakes