IGCSE Physics / Forces and movement
What affects stopping distance
Edexcel International GCSE Physics (4PH1), chapter 3: Forces and movement, specification point 1.20. Free lesson.
Read the revision notes · IGCSE Physics past papers
Stopping distance is the total distance travelled while the driver reacts and while the car brakes. Knowing which factors affect thinking distance or braking distance helps you explain how stopping distance changes.
Which factors affect each distance?
Stopping distance has two parts: thinking distance, travelled while the driver reacts, and braking distance, travelled after the brakes are applied. Speed affects both parts because a faster car travels further during each stage. Other factors affect only one part. Thinking distance: tiredness, alcohol, drugs and distractions make reaction time longer. Braking distance: greater mass, wet, icy or gravel roads, and worn tyres or brakes make it longer. Reaction time does not change braking distance. Fog and darkness delay seeing a hazard. They act before the driver starts…
Explain a thinking-distance factor as a chain
For a thinking-distance question, connect the factor to the driver's reaction time, then to the distance travelled before braking. Join each link with so and finish by naming the distance that increases.
Explain a braking-distance factor as a chain
A road with less grip reduces the force that slows the car. This gives a smaller deceleration, meaning the car slows down less quickly, so it travels further before stopping.
How mass affects braking distance
For the same braking force, a vehicle with greater mass has a smaller deceleration. Use F = m × a to connect mass and acceleration, then state the effect on braking distance. A fully loaded van has greater mass. With the same braking force, its deceleration is smaller. It takes a greater distance to stop, so its braking distance is longer.
Keep reaction time separate from braking
Alcohol affects the driver before the brakes are applied. It slows reactions, so it increases thinking distance, not braking distance. Once the brakes are on, the car slows just the same. Speed increases both thinking distance and braking distance. Tiredness, alcohol, drugs and distractions increase thinking distance. Mass, road surface, tyres and brakes affect braking distance. For a wet, icy or gravel road, less friction leads to a smaller braking force and deceleration. Both braking distance and stopping distance increase.