Forces, Movement & Changing Shape
Forces & Motion · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.
Definition and breakdown
- Stopping distance is the total distance a vehicle travels from the instant the driver spots a hazard to the instant the vehicle finally comes to rest
- It splits into two stages:
stopping distance = thinking distance + braking distance
- Thinking distance = the distance the vehicle covers during the driver's reaction time, before the brakes are even pressed
- Braking distance = the distance the vehicle covers while it is decelerating under the braking force, from the moment the brakes are first applied to the moment it stops
Reaction time and thinking distance
- Reaction time is the time gap between a driver noticing a hazard and starting to act on it
- A typical adult driver's reaction time is about 0.7 s (roughly two-thirds of a second) when alert and undistracted. This is the value the UK Highway Code uses for its stopping-distance tables
- Thinking distance grows with reaction time and with vehicle speed:
- longer reaction time → more time before the brakes are pressed
- higher speed → more metres covered during that reaction time
- Reaction time is lengthened by:
- tiredness
- distractions such as using a phone, eating, or fiddling with controls
- alcohol or drugs
Braking distance
- The braking distance counts every metre the vehicle still rolls forward from the moment the brakes first bite until it finally comes to rest
- For a given braking force, braking distance grows with:
- vehicle speed, because a faster vehicle has more kinetic energy that the brakes must remove
- vehicle , because heavier vehicles need more work done by the brakes to decelerate
- road conditions, because wet, icy or oily roads reduce the friction between tyre and road, lengthening the braking distance
- brake and tyre condition, because worn brakes or bald tyres also lengthen braking distance
Speed dominates
| Speed / mph | Speed / m/s | Approximate stopping distance / m |
|---|---|---|
| 20 | 9 | 12 |
| 30 | 13 | 23 |
| 40 | 18 | 36 |
| 50 | 22 | 53 |
| 60 | 27 | 73 |
- Doubling the speed roughly quadruples the braking distance because braking distance depends on the square of the speed (consistent with v² = u² + 2as)
Effect of a factor on thinking distance vs braking distance
What comes up: "Explain the effect, if any, of [factor] on the thinking distance and on the braking distance."
Write (two marks each component, four marks total): For thinking distance — state whether it increases or decreases, then link it to a change in the driver's reaction time. For braking distance — state there is no effect, and explain that braking distance depends on the braking force and road conditions, not on the driver's reaction time.
Watch out: tiredness lengthens reaction time and therefore increases thinking distance, but has no effect on braking distance. Confusing the two components in an answer (saying tiredness increases braking distance) scores zero for the braking distance part.