1 · IGCSE Physics
Stopping distance
Watch a car react to a hazard and brake to rest. Change the speed, reaction time, road surface and mass to see how thinking distance and braking distance each change.
thinking = v × tᵣ
braking = v² ÷ (2a)
a = F ÷ m, F = smaller of
8000 N (brakes) and μmg
stopping = thinking + braking
braking = v² ÷ (2a)
a = F ÷ m, F = smaller of
8000 N (brakes) and μmg
stopping = thinking + braking
Try this
- Change the speed from 15 m s⁻¹ to 30 m s⁻¹. Check that the thinking distance doubles but the braking distance becomes four times larger.
- Raise the reaction time from 0.7 s to 1.4 s (a tired or distracted driver). Which distance changes, and which stays the same?
- On a dry road, increase the mass from 800 kg to 2000 kg and watch the braking distance. Then switch to icy and repeat: why does mass now make no difference?
Common error: saying that doubling the speed doubles the stopping distance — only the thinking distance doubles, while the braking distance (∝ v²) becomes four times larger.