Stopping Distances

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CognitoStopping distances and factors affecting them

Key facts & equations

Stopping distance components

Stopping distance = thinking distance + braking distance.
Thinking distance: distance travelled during driver reaction time.
Thinking distance = speed × reaction time.
Average reaction time: ~0.7 seconds.
Braking distance: distance travelled while brakes are applied, until the vehicle stops.
Typical stopping distances: 30 mph (13 m/s) → 23 m. 70 mph (31 m/s) → 96 m.

Factors affecting thinking distance

Higher speed → longer thinking distance (same reaction time, more distance covered).
Factors that INCREASE reaction time: tiredness, alcohol, drugs, distractions (phone), illness.
AQA exam tip: reaction time affects thinking distance only, not braking distance.

Factors affecting braking distance

Higher speed → much longer braking distance (braking distance ∝ v²).
Poor road conditions: wet, icy, greasy roads — reduced friction → longer braking distance.
Poor tyre condition: worn tyres reduce friction.
Poor brakes: brake pads worn → reduced braking force.
Higher mass of vehicle → more kinetic energy to remove → longer braking distance.

Work done and energy in braking

Braking converts kinetic energy to thermal (heat) energy in brakes.
Work done by brakes = braking force × braking distance.
Work done = change in kinetic energy: F × d = ½mv².
Large braking force → short braking distance (same KE removed). But: too large → skidding.
ABS (anti-lock braking systems) prevent wheel lock to maintain maximum friction.

Exam questions — 4 questions · 11 marks · AQA 8463 style

1 markThinking distance factorAQA 8463 style

Which factor affects thinking distance but NOT braking distance?

A Wet road conditions

B Worn brake pads

C Driver reaction time affected by tiredness

D Mass of the vehicle

3 marksCalculate stopping distanceAQA 8463 P1 style 🔢 Calculator

A car travels at 20 m/s. The driver has a reaction time of 0.6 s. The braking distance at this speed is 40 m. Calculate: (a) thinking distance, (b) total stopping distance. (2 marks)

Hint: (a) thinking distance = v × t_reaction. (b) Add both distances.

+30 XP

4 marksBraking energy calculationAQA 8463 Higher style 🔢 Calculator

A car of mass 1200 kg is travelling at 15 m/s. The driver applies brakes with a constant force of 3375 N. Calculate the braking distance. (use F × d = ½mv²) (3 marks)

Hint: Step 1: calculate KE = ½mv². Step 2: rearrange F×d = KE to get d = KE/F.

+40 XP

1 markIce effectAQA 8463 style

Why does driving on ice significantly increase braking distance?

A Ice makes the brakes less effective

B Ice increases the mass of the vehicle

C Ice reduces friction between tyres and road, so less braking force can be applied without skidding

D Ice causes the engine to generate more power

Stopping Distances

Stopping distance components

Factors affecting thinking distance

Factors affecting braking distance

Work done and energy in braking

Module complete! 🎉