Factors Affecting Stopping Distance (OCR GCSE Physics A (Gateway))

Factors Affecting Stopping Distance

• The stopping distance is the distance travelled by a vehicle from the moment the driver acknowledges the need to stop to the vehicle coming to a complete stop

• It is determined by a combination of the braking distance and the thinking distance

Braking Distance

• The braking distance is defined as:

  The distance travelled by a car under the braking force - i.e. whilst it is slowing down

• The main factor affecting the braking distance of a car is its speed

  • The greater the speed, the greater the braking distance will be

• There are additional factors which affect the braking distance, such as:

  • Vehicle condition - e.g. worn tyres or poor brakes

  • Road condition - wet or icy roads make it harder to decelerate

  • Vehicle mass - a heavy vehicle, such as a lorry, takes longer to stop

• The smoother the road conditions, for example when they are wet and icy, the less friction there is between the tyres and the road surface so there would be a greater braking distance

• The braking distance is the ratio of the kinetic energy of the car and the braking force

  • This is because the work done in bringing a car to rest is the transfer of all its kinetic energy into other forms (thermal, sound)

• The kinetic energy is equal to

KE = ½mv²

• This means the braking distance is proportional to the velocity squared

  • If the velocity doubles, the braking distance increases by (2)² , 4 times!

Thinking Distance

• The thinking distance is defined as:

  The distance travelled by a car from when a driver realises they need to brake to when they apply the brakes

• The reaction distance is equal to:

Reaction Distance = Speed of the car × Driver’s reaction time

• The main factor that affects the thinking distance is the car’s speed, however additional factors can affect the thinking distance

• It is increased by:

  • Tiredness

  • Distractions (e.g. using a mobile phone)

  • Intoxication (i.e. consumption of alcohol or drugs)

• Since these factors can affect the driver's reaction time, they directly affect the thinking distance

Dangers of Large Decelerations

• A large declaration 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)

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