5.6.2Forces

Reaction Time — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

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📚 AQA GCSE Combined Science Trilogy 8464 - Physics Hub / Topical Revision

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Section 1 of 9Reaction time and what affects it

Reaction time is the time taken between a stimulus and the response. It depends on how quickly impulses travel along neurones, how long it takes to cross synapses, and how long the brain takes to decide on and start the response. Reaction time is relevant to everyday safety, such as drivers braking in an emergency.

Many factors can alter reaction time, including caffeine (a stimulant), alcohol and other drugs (depressants), tiredness, age, distraction and practice. This required practical investigates the effect of one such factor on a voluntary response.

Section 2 of 9The ruler-drop method

A common method uses a metre ruler.

A partner holds a 30 cm ruler vertically so that the zero end is level with the top of your thumb and finger, which are held open ready to catch.

Without warning the partner lets go of the ruler.

You catch it as quickly as possible, and the distance (cm) at the top of your thumb is recorded.

The test is repeated, at least ten times, and a mean is worked out.

The independent variable is changed (for example, repeat after drinking a caffeinated drink or using the non-dominant hand) and the test repeated.

A shorter distance means a faster reaction. A conversion chart or the equation for free fall can turn distance into time, but distance itself is a valid measure to compare.

An alternative is a computer-based test where the person clicks as soon as a screen changes colour; this gives a direct time in milliseconds and removes human error from timing.

Section 3 of 9Variables and controls

Type	Example
Independent variable	Hand used, or caffeine consumed or not
Dependent variable	Distance the ruler falls (cm), or reaction time
Control variables	Same person, same ruler, same starting position, same time of day, same dropper, same level of distraction

The person releasing the ruler should not give warning or the person catching will anticipate it. The person being tested must not look at the dropper's hand. If testing the effect of practice, the same person is used with repeated trials and the results are plotted against the trial number.

Section 4 of 9Analysing results

Calculate the mean of the repeated results, ignoring anomalies, and compare the means for each condition.

Analysing results

When the first few attempts are much slower than later ones this shows the effect of practice, so record these separately.

Plot a bar chart for categorical conditions (e.g. left or right hand), or a line graph for a continuous variable such as trial number.

For example, if the mean fall with the dominant hand is 12 cm and with the non-dominant hand 17 cm, the dominant hand has the faster reaction, a difference of 5 cm or 42% longer fall for the weaker hand.

Section 5 of 9Evaluation and safety

Sources of error include differences in how the ruler is released, anticipating the drop, and fatigue of the person caught.

Evaluation and safety

Improve reliability by using several people and many repeats, and improve accuracy by using a computer with timing in milliseconds.

Ethical care is needed: do not give caffeine to those who could be harmed, and get consent from participants.

Individual differences mean a large sample is needed before concluding anything about people in general.

The reaction involves the nervous system pathway of receptor, sensory neurone, brain, motor neurone and effector (5.1.1), unlike a reflex (5.1.2).

Section 6 of 9Presenting results as a data table

Record each trial in a results table with a column for the condition, columns for repeats and a column for the mean.

Presenting results as a data table

For example, with a caffeine investigation the trials might be 11, 10, 12, 11 cm without caffeine and 9, 8, 10, 9 cm after it.

The means are 11 cm and 9 cm.

Conclude that the caffeine shortened the fall distance, and so shortened the reaction time,

but note that with only one person tested the conclusion cannot be generalised.

If the results are close together it may mean the difference is within normal variation.

Presenting results as a data table

State the range as well as the mean and say whether the ranges overlap; overlapping ranges mean the evidence for a real difference is weak.

Suggest improvements: more participants, a larger number of repeats, using a blind test where the person does not know whether they have had caffeine,

and using a more accurate measurement tool.

Section 7 of 9Exam-style questions

A student measures the distance a ruler falls before being caught: 14, 12, 13, 25, 11 cm. Calculate the mean, ignoring the anomaly. [2]2 marks
Give three variables that should be controlled in a reaction time investigation. [3]3 marks
A student's results show that reaction time improves with repeated tests. Suggest an explanation. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Reaction time	The time between a stimulus and a response.
Independent variable	The variable you deliberately change.
Dependent variable	The variable you measure.
Control variable	A variable kept the same to make the test fair.
Mean	The sum of the values divided by the number of values.
Anomaly	A result that does not fit the pattern.
Reliable	Giving similar results when the investigation is repeated.

Section 9 of 9Exam technique

Using a single result rather than repeating and calculating a mean.

Changing more than one variable at a time, so the cause of a difference is unclear.

Saying a larger ruler drop means a faster reaction. The longer the fall, the slower the reaction.

Describing reaction time as the same as a reflex. It involves a voluntary decision.

Forgetting to remove anomalies before calculating the mean.

Last updated: 20 October 2026