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Physics · Forces
Stopping distance and reaction time
Define stopping distance as thinking distance plus braking distance, describe factors affecting reaction time, and use \(s = vt\) for thinking distance.
Teacher resources
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- Stopping distance and reaction time - Teacher Slides.pptx Teacher The lesson slides with the teacher's notes on each slide, and every question and mark scheme built in. Built from the lesson script on 30 September 2026. View
- Stopping distance and reaction time - Teacher Notes.docx Teacher The complete notes with the teacher's notes and every model answer in full. Built from the lesson script on 30 September 2026. View
Student handouts
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- Stopping distance and reaction time.pptx Built from the lesson script on 30 September 2026. View
- Stopping distance and reaction time - Completed Notes.docx The full notes for the lesson, to revise from. Built from the lesson script on 30 September 2026. View
- Stopping distance and reaction time - Exam Questions.docx Built from the lesson script on 30 September 2026. View
Warm-up
Answer each one, then check.
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1
What is speed?
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Distance divided by time
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2
What is distance = speed × time?
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\(s = vt\)
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3
What is reaction time?
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The time between seeing something and reacting
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4
What does braking do?
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Reduces the vehicle's kinetic energy
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5
What is a typical human reaction time?
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0.2 s to 0.9 s
Learning Objectives
- 1Define stopping distance, thinking distance and braking distance.
- 2Explain how speed affects stopping distance.
- 3Describe factors that affect reaction time, and methods used to measure it.
- 4Interpret graphs relating speed to stopping distance.
STOPPING DISTANCE
stopping distance = thinking distance + braking distance
Thinking distance is the distance travelled during the driver's reaction time. Braking distance is the distance travelled under the braking force. For a given braking force, the greater the speed, the greater the stopping distance.
Stopping Distance
Thinking distance is proportional to speed; braking distance is not.
Reaction Time
Typical values and factors.
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Typical reaction time
Detail: 0.2 s to 0.9 s
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Increases with
Detail: Tiredness, drugs, alcohol, distractions such as a mobile phone
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Thinking distance
Detail: \(s = vt\): reaction time × speed
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Measuring it
Detail: Ruler drop test: the distance the ruler falls gives the reaction time
Thinking Distance
A driver travelling at 15 m/s has a reaction time of 0.60 s. Calculate the thinking distance.
Show the solutionHide the solution
- 1 Write the equation \(s = vt\)
- 2 Substitute \(s = 15 \times 0.60\)
- 3 Answer \(s = 9.0\) m
Answer9.0 m
Stopping Distance
A car has a thinking distance of 9.0 m and a braking distance of 14 m. Calculate the stopping distance.
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- 1 Add \(9.0 + 14 = 23\) m
Answer23 m
Measuring Reaction Time
Describe the ruler drop method.
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- 1 Set up A partner holds a ruler vertically with the zero between your thumb and finger
- 2 Drop Without warning they drop it, and you catch it as fast as you can
- 3 Measure The distance fallen is read from the ruler
- 4 Convert Use a table of distance against time, then repeat and find a mean
AnswerThe longer the distance the ruler falls, the longer the reaction time.
Distractions
Explain the effect of a driver texting on the stopping distance.
Show the solutionHide the solution
- 1 Reaction time Increases
- 2 Thinking distance \(s = vt\) is greater
- 3 Stopping distance So the total stopping distance is greater
AnswerA longer reaction time gives a longer thinking distance and so a longer stopping distance.
Why It Matters
Safety.
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Speed limits
Higher speed means a much greater stopping distance.
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Driver factors
Tiredness, alcohol, drugs and distractions raise reaction time.
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Estimates
Estimate how emergency stopping distance varies over typical speeds.
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Graphs
Interpret graphs of speed against stopping distance for different vehicles.
Reaction Test
In a ruler drop test a student's three results for the distance fallen are 14 cm, 16 cm and 15 cm. Calculate the mean. Explain why a tired driver would have a longer stopping distance.
1. Find the mean.
2. Link to s = vt.
A good answer shows: Mean = 15 cm. Tiredness increases reaction time, so thinking distance increases, increasing the stopping distance.
Can I...?
- 1Define stopping distance.
- 2Define thinking and braking distance.
- 3Use s = vt.
- 4List factors affecting reaction time.
- 5Describe a reaction-time test.
- 6Read a stopping distance chart.
- 7Explain the effect of speed.
- 8Estimate stopping distances.
Summary & Exam Focus
- Stopping = thinking + braking.
- Thinking distance: \(s = vt\).
- Reaction time 0.2 to 0.9 s; increased by tiredness, drugs, alcohol, distractions.
- Higher speed gives a greater stopping distance.
Exam focus
A driver travels at 15 m/s with a reaction time of 0.60 s. Calculate the thinking distance. (2 marks) (2 marks)
Multiply speed by reaction time.
Key terms
The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.
- Stopping distance
- Thinking distance plus braking distance.
- Thinking distance
- Distance travelled in the driver's reaction time.
- Braking distance
- Distance travelled while the brakes slow the vehicle.
- Reaction time
- The time between a stimulus and a response.
- Distraction
- Something that takes a driver's attention away.
- Mean
- The average of repeated readings.
Questions and answers
10 questions set on this lesson, with the mark schemes and model answers open.
A car travels at 15 m/s. The driver's reaction time is 0.60 s. Calculate the thinking distance. Use the equation: distance = speed × time
Mark scheme — 2 marks available
- Correct substitution — 1 mark
- 9.0 m — 1 mark
Model answer
\(15 \times 0.60 = 9.0\) m
Give two factors that can increase a driver's reaction time.
Mark scheme — 2 marks available
- First factor — 1 mark
- Second factor — 1 mark
Model answer
Any two from: tiredness, alcohol, drugs, distractions (such as using a mobile phone).
The chart shows the thinking distance and braking distance for a car at different speeds. (a) Use the chart to find the stopping distance at 50 mph. (b) Describe how the thinking distance changes with speed. (c) Explain why the braking distance increases more quickly than the thinking distance.
Mark scheme — 4 marks available
- 53 m — 1 mark
- Thinking distance increases steadily or proportionally — 1 mark
- Braking distance depends on kinetic energy — 1 mark
- Kinetic energy ∝ speed² — 1 mark
Model answer
(a) About 53 m. (b) The thinking distance is directly proportional to the speed (it increases by equal amounts). (c) The braking distance depends on the kinetic energy of the car, which increases with the square of the speed.
A driver is sending a text message while driving. Explain how this affects the stopping distance of the car.
Mark scheme — 3 marks available
- Reaction time increases — 1 mark
- Thinking distance increases — 1 mark
- Stopping distance greater — 1 mark
Model answer
The driver's attention is on the phone so their reaction time is longer. Thinking distance = speed × reaction time, so the thinking distance increases and the stopping distance is greater.
Describe an experiment to measure a person's reaction time using a ruler. Explain how the result is used.
Mark scheme — 4 marks available
- Ruler held and dropped without warning — 1 mark
- Distance fallen measured — 1 mark
- Converted to a reaction time — 1 mark
- Repeat and mean, compare conditions — 1 mark
Model answer
A partner holds a ruler vertically with zero level with the person's thumb and finger. The ruler is dropped without warning and caught. The distance fallen is read off and converted to a time (from a table or calculation). The test is repeated and a mean found; different conditions (distractions) can be compared.
Stopping distance is...
Why: The sum of the two.
A typical reaction time is...
Why: Typical human range.
Which does NOT increase reaction time?
Why: Water is not a cause.
A longer reaction time gives a...
Why: s = vt.
At higher speed the stopping distance is...
Why: Both components increase.