AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Forces and braking
Practise Reaction time. 11 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recall typical reaction timesTypical reaction times range from 0.2 s to 0.9 s.
4
List factors that increase reaction timeTiredness, drugs, alcohol and distractions (e.g. using a phone).
Describe the ruler-drop reaction time testCatch a dropped ruler; the further it falls before you catch it, the longer your reaction time.
6
Evaluate reaction time measurementsIdentify anomalies, calculate means and suggest how to make a test fairer.
7
Evaluate factors affecting thinking distance from dataCompare thinking distances with and without a factor, at the same speed.
8
Calculate a reaction time from ruler-drop distanceUse v2 − u2 = 2 a s with a = 9.8 m/s2 to find v, then t = v ÷ a.
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
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 further the ruler falls, the longer your reaction time. A conversion table or graph turns the distance into a time.
You can also calculate the time: the ruler falls from rest with an acceleration of 9.8 m/s2, so use v2 = 2 × 9.8 × distance, then t = v ÷ 9.8. grade 8+
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.
Cheatsheet
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
Ruler drop calculation: v2 = 2 a s with a = 9.8 m/s2, then t = v ÷ a grade 8+
How to answer each type of question
Recall typical values and factors
3 marks3
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.
Show the model answer
(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 marks5
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.
Show the model answer
normal: 20 × 0.50 = 10 m (1) using a phone: 20 × 0.80 = 16 m (1) increase = 6.0 m (1)
Describe how to measure reaction time
4 marks6
Describe the starting position of the ruler and hand.
Say that the ruler is dropped without warning and caught.
Say what is read and how it becomes a time.
Say how the result is made more reliable.
Example. Describe how a student can use a metre ruler to measure a partner's reaction time.
Show the model answer
The partner holds their finger and thumb open, level with the zero mark of the ruler held vertically above (1). The student drops the ruler without warning (1). The partner catches it and the distance fallen is read from the ruler (1). A conversion table (or graph) gives the reaction time; repeat several times and find the mean (1).
Evaluate reaction time data
3 marks7
Identify any result that does not fit the pattern (anomaly).
Leave it out and calculate the mean of the rest.
Round to a sensible number of significant figures.
Example. A student's reaction times from four ruler-drop tests were 0.21 s, 0.24 s, 0.46 s and 0.22 s. (a) Identify the anomalous result. (b) Calculate the mean reaction time, ignoring the anomalous result.
Calculate a reaction time from a ruler-drop distance
3 marks8
The ruler starts from rest, so u = 0 and a = 9.8 m/s2.
Use v2 − u2 = 2 a s to find v.
Use a = Δv ÷ t, so t = v ÷ a.
Example. In a ruler-drop test, the ruler falls 0.18 m before it is caught. Use the equation v2 − u2 = 2 a s, with a = 9.8 m/s2, to calculate the reaction time.
Show the model answer
v2 = 0 + 2 × 9.8 × 0.18 = 3.528 (1) v = 1.88 m/s (1) t = 1.88 ÷ 9.8 = 0.19 s (1)
Shortcuts and 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.
Where marks are lost
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.
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.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Give two factors that can increase a driver’s reaction time.
Any two of: tiredness, alcohol, drugs, distractions.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Which of these is a typical reaction time for a driver? Tick (✓) one box.[1]
0.02 s
0.5 s
2.5 s
5.0 s
(b) Give two factors that can increase a driver’s reaction time.[2]
(c) A driver has a reaction time of 0.60 s and is travelling at 15 m/s. Calculate the thinking distance. Use the equation: distance = speed × time[2]
Show the answer and mark scheme
(a)Answer: 0.5 s
(b)Answer: Any two of: tiredness, alcohol, drugs, distractions.
tiredness
drinking alcohol
taking (some) drugs
distractions, e.g. using a mobile phone
(c)Answer: 9.0 m
s = 15 × 0.60
9.0 (m)
Question 2Medium5 marks
A student carries out the ruler-drop test to measure their reaction time.
(a) Explain why the student should repeat the test several times and calculate a mean, rather than relying on a single reading.[2]
(b) The student’s five results are 0.18 s, 0.21 s, 0.19 s, 0.55 s and 0.20 s. Identify the anomalous result.[1]
(c) Calculate the mean of the remaining results.[2]
Show the answer and mark scheme
(a)Answer: A single reading may be untypical or anomalous; a mean of repeats reduces the effect of random variation.
a single reading could be affected by random variation (for example a lapse in concentration), so it may not be typical / could be anomalous
repeating and calculating a mean reduces the effect of random variation, giving a more reliable value
(b)Answer: 0.55 s.
0.55 s
(c)Answer: 0.20 s (0.195 s) s
(0.18 + 0.21 + 0.19 + 0.20) ÷ 4
0.20 (s) (0.195)
Question 3Hard7 marks
Some students think that talking on a hands-free phone increases a person’s reaction time.
(a) Plan an investigation, using the ruler-drop test, to find out whether talking on a hands-free phone affects a person’s reaction time.[6]
(b) Suggest why person B should not be able to see person A’s hand before the ruler is dropped.[1]
Show the answer and mark scheme
(a)
person A holds a metre rule vertically with the zero mark level with the top of person B’s thumb, with B’s thumb and finger either side of (not touching) the ruler
person B rests their forearm on the edge of a table so that the hand is in a fixed position
person A drops the ruler without warning and person B catches it as quickly as possible
record the distance the ruler falls, read at the top of the thumb
repeat several times (e.g. 10) and calculate a mean, ignoring anomalies
repeat the whole test while person B is talking on a hands-free phone
control variables: same catcher, same hand, same ruler, same starting position, same person dropping
convert distances to reaction times using a conversion table (or t = √(2d ÷ g))
compare the mean reaction times with and without the phone call
Marked with levels of response: the full level descriptors are in the app.
(b)Answer: So B reacts to the ruler moving rather than anticipating the drop.
so that person B reacts to the ruler moving and cannot anticipate the drop