Factors Affecting Braking Distance (AQA GCSE Physics)

• The braking distance is 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
• 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

• When a driver applies the brakes, there is a frictional force between the brakes and the wheels of the car
• This frictional force does work on the brakes - i.e. it transfers energy from the car to the brakes
• Therefore, the kinetic energy of the car decreases and the thermal energy of the brakes increases - i.e. the brakes heat up
• This means the car decelerates (slows down)

Work done by breaking transfers kinetic into thermal energy

• The greater the speed of a vehicle, the greater the braking force required to bring the vehicle to a halt for a given distance
• Since the braking force would need to be larger, the deceleration of the vehicle will be large as well
  • This is due to the link between resultant force and acceleration as stated in Newton's second law of motion

• Large decelerations could lead to the brakes overheating and / or loss of control of the vehicle

Part (a)

Step 1: Recall the process of applying brakes to a vehicle

• • The work done is the energy transferred from the car and driver to the brakes
  • This energy transfer is from kinetic energy to thermal energy
  • In this case, the car is brought to a complete stop, so 168 750 J of energy is transferred from kinetic energy to heating up the brakes (assuming all energy is transferred to heat only!)

Part (b)

Step 1: State the equation for work done

Work done = Force × distance travelled (or W = Fs)

• • In this case, the force doing the work (transferring energy) is the braking force

Step 2: Rearrange the equation and solve for distance travelled

Distance travelled = Work done ÷ Force

Step 3: Substitute the values for work and force 

Distance = 168 750 ÷ 6000 = 28.1 m

• • Hence the braking distance (the distance travelled by the car under the braking force) is 28.1 m