OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) reaction time and thinking distance; braking distance and stopping…
Full specification wording
Linear motion: (c) reaction time and thinking distance; braking distance and stopping distance for a vehicle.
Stopping distance is the total distance a vehicle travels from the moment the driver needs to stop until it is at rest.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Stopping distance is the total distance a vehicle travels from the moment the driver needs to stop until it is at rest. It has two parts. Thinking distance is the distance travelled during the driver's reaction time, before the brakes are applied; it equals speed multiplied by reaction time, so it grows with speed and with a slower reaction. Braking distance is the distance travelled while the brakes are applied, and it depends on the braking force, the mass of the vehicle and the road conditions. Stopping distance is the sum of the two. Because thinking distance is proportional to speed and braking distance depends on the square of speed, stopping distance rises steeply as speed increases.
What earns the marks
- Thinking distance is the distance travelled during the driver's reaction time, before the brakes are applied.
- Thinking distance equals speed multiplied by reaction time, so it increases with speed and with a longer reaction time.
- Braking distance is the distance travelled while the brakes are applied, and it depends on braking force, vehicle mass and road conditions.
- Stopping distance is the sum of thinking distance and braking distance, and it increases more than proportionally with speed.
Common mistakes and how to put them right
Confusing thinking distance with braking distance; thinking distance occurs before the brakes act, while braking distance occurs while they act.
Assuming stopping distance is proportional to speed; thinking distance is proportional to speed but braking distance depends on speed squared, so the total rises steeply.
Forgetting that reaction time varies with the driver's state, such as tiredness or alcohol, which changes thinking distance without changing the braking system.
Exam tips
- Write the relationship as stopping distance = thinking distance + braking distance before substituting any values.
- Check whether a question gives reaction time or asks you to find it; thinking distance divided by speed gives reaction time.
- When comparing two speeds, remember that doubling the speed doubles thinking distance but roughly quadruples braking distance.
Try the lesson
The start of the Linear motion: (c) reaction time and thinking distance; braking distance and stopping distance for a vehicle. lesson, made from checked teaching notes for this specification point.
Listen
Stopping distance is the total distance a vehicle travels from the moment the driver needs to stop until it is at rest. It has two parts. Thinking distance is the distance travelled during the driver's reaction time, before the brakes are applied; it equals speed multiplied by reaction time, so it grows with speed and with a slower reaction. Braking distance is the distance travelled while the brakes are applied, and it depends on the braking force, the mass of the vehicle and the road conditions. Stopping distance is the sum of the two. Because thinking distance is proportional to speed and braking distance depends on the square of speed, stopping distance rises steeply as speed increases. A strong answer earns marks for these points. Thinking distance is the distance travelled during the driver's reaction time, before the brakes are applied. Thinking distance equals speed multiplied by reaction time, so it increases with speed and with a longer reaction time. Braking distance is the distance travelled while the brakes are applied, and it depends on braking force, vehicle mass and road conditions. Stopping distance is the sum of thinking distance and braking distance, and it increases more than proportionally with speed. Exam tip: Write the relationship as stopping distance = thinking distance + braking distance before substituting any values.
See it
Key words
Say what each word means before you turn the card.
Thinking distance
Try answering aloud or in your head before turning the card.
Braking distance
Try answering aloud or in your head before turning the card.
Stopping distance
Try answering aloud or in your head before turning the card.
Reaction time
Try answering aloud or in your head before turning the card.
See every definition
- Thinking distance
- The distance travelled during the driver's reaction time, before the brakes are applied; it equals speed multiplied by reaction time.
- Braking distance
- The distance travelled while the brakes are applied, depending on the braking force, the mass of the vehicle and the road conditions.
- Stopping distance
- The total distance a vehicle travels from the moment the driver needs to stop until it is at rest, being the sum of thinking distance and braking distance.
- Reaction time
- The time before the brakes are applied, during which the thinking distance is travelled; thinking distance divided by speed gives it.
Label the diagram
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1Box 1
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3Box 3
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