Factors affecting braking distance 2 — AQA GCSE Physics
Test yourself on Factors affecting braking distance 2 with AQA GCSE practice questions.
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Factors affecting braking distance 2 explained
Pressing the brake pedal forces the pads against the discs or drums attached to the wheels.
Read the full explanation
As the wheel turns against the pads, a friction force acts and work is done. That work transfers energy out of the kinetic energy store of the vehicle and into the thermal energy store of the brakes, so the vehicle slows and the brakes get hot. The amount is given by work done = force × distance, and it equals the kinetic energy removed, ½ m v². This is why brake discs glow after a long descent and why repeated heavy braking can overheat them. Saying only 'friction' is not enough in an examination: you must say that work is done. For a 1200 kg car at 20 m/s the brakes must do 240 000 J of work, which is why brake discs can reach several hundred degrees Celsius.
Your focus
- Explain why the temperature of the brakes rises during a stop, naming the energy stores involved.
- Calculate the work done by the brakes from the braking force and the braking distance.
- Determine the braking force needed to stop a vehicle of stated mass and speed in a given distance.
Factors affecting braking distance 2 exam tips
Marking Points
- one mark for stating that work is done against friction between the brake pads and the wheel or disc
- one mark for stating that the kinetic energy store of the vehicle decreases
- one mark for stating that the internal, or thermal, energy of the brakes increases so their temperature rises
- one mark for using work done = force × distance
- one mark for equating the work done to the kinetic energy ½ m v²
Examiner Tips
- 💡Use the phrase 'work is done against friction' rather than 'friction slows it down'.
- 💡Name both stores in the transfer: kinetic store down, thermal store of the brakes up.
- 💡Two-mark questions here usually want one mark for the work done and one for the temperature rise, so give both.
Common Mistakes
- writing only the word friction, which the mark scheme explicitly says is insufficient
- saying heat is created rather than energy being transferred to a thermal store
- describing the kinetic energy as lost or used up
- substituting the whole stopping distance into work done = force × distance rather than the braking distance