What affects how far a car travels before it stops, and how do safety features protect us?
Thinking, braking and stopping distances, the factors affecting each, and how safety features reduce the force in a crash.
A CCEA GCSE Physics answer on thinking, braking and stopping distances, the factors that affect each, and how seat belts, air bags and crumple zones reduce injury by increasing the time of impact.
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What this dot point is asking
CCEA wants you to define thinking, braking and stopping distances, state the factors that affect each, and explain how safety features such as seat belts, air bags and crumple zones reduce the force on people in a crash. The safety explanation links to momentum.
The answer
The three distances
Factors affecting thinking distance
Thinking distance depends on the reaction time and the speed. It increases if:
- the car is going faster (further travelled in the same reaction time);
- the driver is tired, distracted, or under the influence of alcohol or drugs, which lengthens reaction time.
Factors affecting braking distance
Braking distance depends on the speed and on the braking force and friction. It increases if:
- the car is going faster (it rises steeply with speed);
- the road is wet or icy or the tyres or brakes are worn, reducing friction;
- the car is more heavily loaded.
Vehicle safety features
In a crash, a passenger's momentum must fall to zero. Because force is the rate of change of momentum, spreading that change over a longer time gives a smaller force.
Worked example: stopping distance
Examples in context
Example 1. Speed limits near schools. Lower speeds cut both the thinking distance and, much more sharply, the braking distance, so a car can stop in a far shorter total distance, protecting pedestrians.
Example 2. A crash mat for a gymnast. Landing on a thick mat increases the time to stop, reducing the force on the gymnast, exactly the same physics as a car crumple zone.
Try this
Q1. Write the equation linking stopping, thinking and braking distance. [1 mark]
- Cue. Stopping distance = thinking distance + braking distance.
Q2. State two factors that increase thinking distance. [2 marks]
- Cue. Higher speed; longer reaction time from tiredness, distraction or alcohol.
Q3. Explain in one sentence how an air bag reduces injury. [2 marks]
- Cue. It increases the time to stop the passenger, so the force (rate of change of momentum) is smaller.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksExplain what is meant by thinking distance and braking distance, and state two factors that increase the braking distance of a car.Show worked answer →
Thinking distance is the distance travelled during the driver's reaction time, between seeing a hazard and pressing the brake.
Braking distance is the distance travelled while the brakes act, from first braking until the car stops.
Two factors that increase braking distance: higher speed; and reduced friction between tyres and road, for example wet or icy roads (or worn tyres, worn brakes).
Markers reward correct definitions of both distances and two valid factors affecting braking distance.
CCEA style4 marksA car crash test shows that air bags and crumple zones reduce the force on the passengers. Use ideas about momentum and time to explain how they do this.Show worked answer →
In a crash the passenger's momentum must be reduced to zero. The change in momentum is fixed by the speed and mass.
A crumple zone and an air bag increase the time taken to stop the passenger. Because the force equals the rate of change of momentum, increasing the stopping time reduces the force on the passenger.
A smaller force on the body means less injury.
Markers reward: change in momentum fixed; features increase the stopping time; force is rate of change of momentum, so longer time means smaller force; smaller force reduces injury.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)