Stopping DistancesEdexcel GCSE Physics: Revision notes
Section 1
What makes up a vehicle's stopping distance?
Stopping distance = thinking distance + braking distance
- Thinking distance: the distance travelled during the driver's reaction time, before the brakes are applied
- Braking distance: the distance travelled once the brakes are applied, while the vehicle decelerates to a stop
Both distances increase as speed increases, but they don't increase in the same way — thinking distance is proportional to speed, while braking distance increases much more sharply.
Section 2
How is reaction time measured and what affects it?
Human reaction times can be measured with a ruler-drop test: one person drops a ruler without warning and another catches it as quickly as possible; the distance the ruler falls before being caught gives the reaction time (using distance–time relationships for free fall). Typical human reaction times are around 0.2–0.9 seconds.
Factors that increase (worsen) reaction time include:
- Alcohol and drugs (including some medicines)
- Tiredness
- Distractions, such as using a mobile phone or talking to passengers
When asked to explain why thinking distance increases, examiners want: 'reaction time stays roughly the same, but the vehicle covers more distance in that time at higher speed' — not just 'because speed is higher'.
Section 3
What factors affect stopping distance?
Stopping distance is affected by:
- Mass and speed of the vehicle — greater mass or speed increases braking distance
- Driver's reaction time — affects thinking distance (see drugs/distractions above)
- State of the brakes — worn brakes increase braking distance
- State of the road — wet, icy or loose surfaces reduce grip and increase braking distance
- Friction between tyre and road surface — worn tyres or low tread reduce friction, increasing braking distance
Don't confuse thinking distance factors (reaction time — drugs, tiredness, distraction) with braking distance factors (vehicle/road/tyre condition, mass, speed) — examiners often test whether you can sort a list correctly.
Section 4
Why are large decelerations dangerous, and how does braking distance relate to speed?
A large deceleration means a large force is needed to stop the vehicle (F = m × a), which can cause serious injury to occupants and damage on impact. Typical decelerations in a road collision can be tens of m/s², producing forces of many thousands of newtons on a vehicle and its occupants.
Braking distance increases with the square of the initial speed. This can be shown using work done: the kinetic energy of the vehicle (½mv²) must be removed by the work done by the braking force (F × d). Since KE ∝ v², the braking distance d is also proportional to v² — doubling speed roughly quadruples braking distance.
Doubling a car's speed from 20 mph to 40 mph roughly quadruples its braking distance, because braking distance is proportional to speed squared.
Must Know
- Stopping distance = thinking distance + braking distance
- Thinking distance depends on reaction time; braking distance depends on vehicle/road/tyre condition, mass and speed
- Drugs, tiredness and distractions increase reaction time and therefore thinking distance
- Wet/icy roads and worn brakes/tyres increase braking distance
- Large decelerations require large forces (F = m × a) and are dangerous
- Braking distance is proportional to speed squared (from work done = kinetic energy removed)
That's the notes covered.
Carry on to the next subtopic.