2.4.4 Calculating 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:
• • The image below shows how the stopping distance of a typical family car increases with increasing speed:

  
• 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)

  
• • When a vehicle stops work is done by a force
  • The kinetic energy of the car is transferred to thermal energy in the brakes which does work
  • This can also be represented by the braking force and braking distance by the following equation
  • This equation shows that the work done is the transfer of kinetic energy
  • We can use this equation to estimate the decelerating forces required for a typical vehicle moving at everyday speeds

  
• • The equation can be rearranged to show how the braking distance depends on velocity:

  Equation for braking distance from mass, velocity and braking force

  • The braking distance is proportional to the vehicle's velocity squared
  • For example, if the velocity of the vehicle doubles then the braking distance will increase by a factor of 4

  
• Exam Tip

  The equation for braking distance doesn't actually apply at very high speeds because the brakes get hot and become less effective. This reduces the braking force, causing the braking distance to increase even further. This is why it is important to prevent brakes from overheating.