AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Forces › Forces and motion › Forces and braking
Practise Reaction time. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Reaction time is the time between a driver seeing a hazard and acting on it. You need to recall typical values (0.2 s to 0.9 s), the factors that increase reaction time, how reaction time affects thinking distance, and how to measure and evaluate reaction times, for example with a falling ruler.
Key facts
Typical reaction time: 0.2 s to 0.9 s
Reaction time is increased by tiredness, drugs, alcohol and distractions
Ruler drop: the longer the distance fallen, the longer the reaction time
Notes
Reaction time
Reaction time is the time between seeing a hazard (or other stimulus) and responding to it, e.g. pressing the brake pedal.
Reaction times vary from person to person. Typical values range from 0.2 s to 0.9 s.
A driver's reaction time can be increased by tiredness, drugs and alcohol.
Distractions, such as using a mobile phone or talking to passengers, may also affect a driver's ability to react.
Reaction time and thinking distance
During the reaction time, the vehicle keeps moving at its original speed.
Thinking distance = speed × reaction time (s = v t).
A longer reaction time or a greater speed gives a longer thinking distance, and so a longer stopping distance.
Example: at 25 m/s, an increase in reaction time from 0.5 s to 0.9 s increases the thinking distance from 12.5 m to 22.5 m.
Measuring reaction time: the ruler drop
diagram
A partner holds a metre ruler vertically, with the zero mark level with the top of your open finger and thumb.
They drop it without warning. You catch it as quickly as you can and read the distance it fell.
The ruler-drop test: the further it falls before you catch it, the longer your reaction time.
The further the ruler falls, the longer your reaction time. A conversion table or graph turns the distance into a time.
Repeat several times and calculate a mean. Keep the same catching hand, the same starting position and the same person dropping the ruler.
Computer tests, where you click as soon as the screen changes, are another method.
How to answer each type of question
Recall typical values and factors
3 marksGrade 3
Give a value between 0.2 s and 0.9 s.
Choose from tiredness, alcohol, drugs, distractions.
Example. (a) Give a typical value for a person's reaction time. (b) Give two factors that could increase a driver's reaction time.
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(a) any value from 0.2 s to 0.9 s (1) (b) any two from: tiredness / drinking alcohol / taking drugs / distractions such as using a phone (2)
Calculate a change in thinking distance
3 marksGrade 5
Calculate the thinking distance for each reaction time with s = v t.
Subtract to find the change.
Example. A driver's reaction time is normally 0.50 s. While using a phone, it is 0.80 s. Calculate the increase in thinking distance at a speed of 20 m/s.
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normal: 20 × 0.50 = 10 m (1) using a phone: 20 × 0.80 = 16 m (1) increase = 6.0 m (1)
Don’t lose marks
Saying alcohol increases the braking distance. It increases the reaction time, and so the thinking distance.
Giving a reaction time of several seconds. Typical values are less than 1 s.
Including an anomalous result when calculating a mean.
In the ruler drop, letting the catcher see when the ruler is about to be dropped, or changing the starting position of the fingers.
More tips
Memory tricks
Things that slow reactions: tiredness, drugs, alcohol, distractions.
Quick thinking distance: speed in m/s × reaction time in s. At 30 m/s with 0.7 s, it is 21 m.
Reaction time affects the thinking distance only, never the braking distance.
Exam technique
For method questions, say what is measured (the distance fallen), how it becomes a reaction time, and how you make the result more reliable (repeats and a mean).
In 'evaluate' questions, quote numbers from the data and compare like with like, e.g. at the same speed.
Link each factor in a chain: reaction time → thinking distance → stopping distance.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recall typical reaction timesTypical reaction times range from 0.2 s to 0.9 s.
Grade 4
List factors that increase reaction timeTiredness, drugs, alcohol and distractions (e.g. using a phone).