Reaction time — AQA GCSE Physics
Test yourself on Reaction time with AQA GCSE practice questions.
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Reaction time explained
Reaction times are personal: typical values run from about 0.2 s for an alert, practised person to about 0.9 s.
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They can be measured simply by the ruler drop test, where one student releases a metre rule between another's fingers and the catch distance is converted to a time, or more precisely by a computer test that records the delay between a signal and a key press. Both need several trials and a mean, because reaction time varies from attempt to attempt. The effect on the road is large: at 30 m/s a 0.2 s reaction covers 6 m, while 0.9 s covers 27 m, over four car lengths more before the brakes have done anything at all. Because the values vary so much between people, a class set of results should be reported as a mean together with the range.
A driver’s reaction time can be affected by tiredness, drugs and alcohol. Distractions may also affect a driver’s ability to react.
Tiredness, alcohol, other drugs and distractions such as a phone all lengthen a driver's reaction time. The mechanism is the same in each case: the brain takes longer to process the hazard and send the signal to the muscles, so more time passes before the brakes are applied. Since thinking distance = speed × reaction time, the thinking distance grows, and because stopping distance is thinking plus braking distance, the whole stop lengthens. The brakes, tyres and road are untouched, so the braking distance itself is unchanged. Data on alcohol shows two things worth quoting: the mean reaction time rises after drinking, and the increase is not the same for every person. A phone call or a conversation acts in exactly the same way, adding time before the driver's foot reaches the brake pedal.
Your focus
- Recall a typical range of human reaction times and use a value from it in a thinking distance calculation.
- Calculate how much further a car travels before braking when the reaction time rises from 0.2 s to 0.9 s.
- Explain why several trials and a mean are needed when a reaction time is measured.
Show all 6 objectives
- Explain how a distraction lengthens the stopping distance by way of reaction time and thinking distance.
- Draw a conclusion about alcohol from a graph of individual reaction times, referring to both the mean and the spread.
- Sort a list of given factors into those that change thinking distance and those that change braking distance.
Reaction time exam tips
Marking Points
- one mark for quoting a typical reaction time inside the range 0.2 s to 0.9 s
- one mark for using thinking distance = speed × reaction time
- one mark for a correct substitution such as 30 × 0.7
- one mark for the answer in metres
- one mark for naming a factor: tiredness, alcohol, another drug or a distraction
- one mark for stating that the reaction time increases
- one mark for linking that to a longer thinking distance
- one mark for concluding that the stopping distance increases
- one mark for a data conclusion such as the mean reaction time increases after alcohol but not by the same amount for everyone
Examiner Tips
- 💡Learn the range 0.2 s to 0.9 s as a recall fact; questions often ask for a typical value with no data given.
- 💡State the reaction time you are using before the substitution so the examiner can follow it.
- 💡If asked to comment on a measured value, compare it with the typical range rather than calling it right or wrong.
- 💡Check whether the question asks about thinking or braking distance before you choose your factors.
- 💡When a graph of individual results is given, comment on the spread as well as the mean.
- 💡Say 'increases the reaction time' explicitly; naming the factor alone is usually only one of the marks.
Common Mistakes
- quoting a reaction time in milliseconds and then substituting the number without converting to seconds
- giving a reaction time of several seconds, far outside the typical range
- taking a single measurement rather than repeating and finding a mean
- assuming every driver has the same reaction time, so one measured value applies to all
- listing wet or icy roads, which lengthen the braking distance rather than the reaction time
- saying alcohol increases the braking distance
- writing that the driver is 'slower' without naming reaction time or thinking distance
- claiming every person's reaction time rises by the same amount after drinking, which the data does not support