Core Practical: Investigating Reaction Time

This lesson covers the core practical on investigating reaction time using the ruler drop test, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the method, identify variables, calculate reaction time, evaluate results and suggest improvements to accuracy.

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Introduction

Reaction time is the time it takes for a person to respond to a stimulus. It is a measure of how quickly the nervous system can detect a stimulus, process it and produce a response.

Typical human reaction times:

Response Type	Typical Reaction Time
Visual stimulus (e.g. catching a ruler)	200–300 ms (0.2–0.3 s)
Auditory stimulus (e.g. responding to a sound)	150–250 ms
Touch stimulus	150–200 ms

This practical uses the ruler drop test to measure reaction time. It tests the reaction time of the nervous system: Stimulus (ruler drops) → Receptor (eyes detect movement) → Sensory neurone → CNS → Motor neurone → Effector (hand muscles close) → Response (ruler caught).

Exam Tip: This is a core practical — you will be expected to describe the method, identify variables, analyse results and evaluate the procedure. Make sure you can do all of these for the ruler drop test.

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Aim

To investigate the effect of a chosen factor on human reaction time using the ruler drop test.

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Apparatus

• 30 cm ruler (metre ruler can also be used)
• Chair and table
• Calculator (for converting distance to reaction time)
• Data table for recording results

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Method

Step-by-Step Procedure

1. Person A (the tester) holds the ruler vertically with the 0 cm mark at the bottom, between the thumb and index finger of Person B (the test subject).
2. Person B positions their thumb and index finger around the 0 cm mark of the ruler, without touching it. There should be a gap of about 1–2 cm between their fingers and the ruler.
3. Person A drops the ruler without warning at a random time.
4. Person B catches the ruler as quickly as possible by closing their thumb and index finger.
5. The distance the ruler has fallen is read from the scale at the point where Person B catches it (at the top of their thumb).
6. This distance is recorded in a data table.
7. The test is repeated at least 5 times for the same person, and a mean distance is calculated (excluding any anomalous results).
8. The mean distance is converted to reaction time using the formula or a conversion table.

graph TD
    A["Person A holds ruler<br/>0 cm at bottom"] --> B["Person B positions<br/>fingers at 0 cm mark"]
    B --> C["Person A drops ruler<br/>without warning"]
    C --> D["Person B catches<br/>ruler as fast as possible"]
    D --> E["Read distance<br/>where ruler was caught"]
    E --> F["Record distance<br/>and repeat 5+ times"]
    F --> G["Calculate mean<br/>and convert to time"]

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Variables

Identifying and controlling variables is essential for a valid experiment:

Variable Type	Variable	Details
Independent variable	The factor being tested (e.g. caffeine intake, time of day, distractions, practice)	This is what you deliberately change
Dependent variable	The distance the ruler falls (converted to reaction time)	This is what you measure
Control variables	Same ruler, same hand, same starting position, same person catching, no distractions (unless testing this), adequate rest between attempts	These must be kept the same to ensure a fair test

Exam Tip: Always state your independent, dependent and control variables clearly when describing an experiment. Examiners award marks for correctly identifying all three.

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Converting Distance to Reaction Time

The ruler falls under gravity (acceleration ≈ 9.8 m/s²). The distance the ruler falls can be converted to reaction time using the formula:

t = √(2d / g)

Where:

• t = reaction time (seconds)
• d = distance fallen (metres — remember to convert from cm)
• g = acceleration due to gravity (9.8 m/s²)

Conversion Table

Distance Fallen (cm)	Reaction Time (s)
5	0.10
10	0.14
15	0.17
20	0.20
25	0.23
30	0.25

Worked Example:

A student catches the ruler at 18 cm (0.18 m).

t = √(2 × 0.18 / 9.8)

t = √(0.36 / 9.8)

t = √(0.0367)

t = 0.19 s (to 2 significant figures)

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Recording and Analysing Results

Sample Results Table

Attempt	Distance (cm)
1	22
2	18
3	20
4	45 (anomalous — not included)
5	19
Mean	(22 + 18 + 20 + 19) ÷ 4 = 19.75 cm

Identifying Anomalous Results

The result of 45 cm is much higher than the others and is likely an anomalous result (an outlier). This could have been caused by:

• The person being distracted.
• A delayed release.
• An error in reading the ruler.

Anomalous results should be identified and excluded from the mean calculation, but you should record them in your data table and note that they were excluded.

Exam Tip: When calculating a mean, always state that anomalous results have been excluded and explain why. Simply ignoring data without justification will lose marks.

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Improving Accuracy and Reliability

Improving Reliability

• Repeat the experiment at least 5 times and calculate a mean — this reduces the impact of random errors and anomalous results.
• Use multiple test subjects — this makes the results more generalisable.
• Ensure the same tester drops the ruler each time to reduce variation.

Improving Accuracy

• Ensure the ruler is held vertically and at the correct starting position (0 cm).
• Avoid giving any visual or verbal cues that the ruler is about to be dropped (Person A should vary the wait time).
• Person B should look at a fixed point on the ruler (e.g. the 0 cm mark) rather than at Person A's hand.
• Use a more precise measuring device or a computer-based reaction time test for more accurate results.

Sources of Error

Error	Type	How to Minimise
Person A gives unconscious cues before dropping	Systematic	Vary the wait time; Person B looks at ruler, not tester
Ruler caught at an angle; reading error	Random	Ensure ruler is vertical; read at top of thumb
Person B anticipates the drop	Systematic	Vary time interval; exclude obviously early catches
Distractions in the room	Random	Conduct in a quiet, controlled environment
Different grip widths between attempts	Random	Keep grip width consistent

Exam Tip: In evaluation questions, always distinguish between random errors (vary unpredictably) and systematic errors (consistently shift results in one direction). Suggest specific improvements for each error you identify.

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Extending the Investigation

The basic experiment can be extended to investigate factors that affect reaction time: