Knowledge Organiser: Stopping Distances
Key Terms
- Thinking distance: Distance during reaction time
- Braking distance: Distance while brakes applied
- Stopping distance: Thinking + braking distance
- Reaction time: Time to respond to hazard
Key Facts
- Thinking distance ∝ v (linear)
- Braking distance ∝ v² (squared)
- Thinking: affected by driver (drugs, fatigue)
- Braking: affected by car/road (tyres, conditions)
Key Equations
- Stopping = Thinking + Braking
- Thinking distance = speed × reaction time
- Braking ∝ v² (from KE = ½mv²)
Exam Tips
- Doubling speed: thinking ×2, braking ×4
- Always link braking distance to KE = ½mv²
- State "driver" factors for thinking, "car/road" for braking
- Wet road → more braking distance (less friction)
Common Mistakes
- Thinking doubling speed doubles braking distance: Braking distance is proportional to v² — doubling speed quadruples braking distance (because KE = ½mv²)
- Confusing thinking distance and braking distance factors: Thinking distance is affected by driver factors (tiredness, drugs, distractions); braking distance is affected by vehicle and road conditions (tyres, brakes, road surface)
- Not linking braking distance to kinetic energy: Examiners expect you to state that larger KE requires more work done by brakes to stop — mention W = F × d and KE = ½mv²
- Saying wet roads affect thinking distance: Wet or icy roads affect braking distance (less friction), not thinking distance — thinking distance depends only on reaction time
- Forgetting large decelerations are dangerous: A very large braking force causes large deceleration — this can cause skidding and loss of vehicle control
Memory Aid
- Thinking distance ∝ v (speed doubles → thinking distance doubles) — linear relationship
- Braking distance ∝ v² (speed doubles → braking distance ×4) — squared relationship