2.4.1 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
• • A large deceleration can be dangerous for a number of different reasons
  • For any object to decelerate, a force is required and for heavy objects, large decelerations will require large forces
  • This is true according to Newton’s Second Law:

  F = ma

  • Where:
  • F = force in newtons (N)
  • m = mass in kilograms (kg)
  • a = acceleration in metres per second squares (m/s2)
• Overheating of Brakes

  • Vehicles use brakes to decelerate
  • These reduce the speed of the vehicle by creating a friction force between the brake and the wheel
  • The kinetic energy of the vehicle is converted to thermal energy of the brakes
  • If brakes get too hot then they can fail, meaning that they will not work effectively the next time they are used
• Work done by breaking transfers kinetic into thermal energy

  
• Loss of Control and Injury

  • When a vehicle undergoes a deceleration then the driver and passengers also experience a deceleration
  • This can cause injuries such as whiplash is a neck injury caused when a persons head moves suddenly, relative to their body
  • It is also more difficult to control a vehicle that is decelerating
  • Losing control can cause a collision
• Exam Tip

  Remember that resultant force is a vector quantity. Examiners may ask you to comment on why its value is negative - this happens when the resultant force acts in the opposite direction to the object's motion. In the worked example above, the resultant force opposes the passenger's motion, slowing them down (decelerating them) to a halt, this is why it has a minus symbol.