In 1796 Nevil Maskelyne, the Astronomer Royal at Greenwich, dismissed his assistant David Kinnebrook. Kinnebrook timed stars crossing a line in the telescope, and his times had drifted to about 0.8 seconds later than Maskelyne's own. Decades later the German astronomer Friedrich Bessel took the case up and found that experienced observers differed from one another by fairly consistent amounts, which became known as the "personal equation". Historians now think the story is more complicated than Maskelyne's own account, but the point stands: everyone has a reaction time, and it can be measured.
This lesson covers the reaction time required practical for AQA GCSE Biology Topic 5 (Homeostasis and response): plan and carry out an investigation into the effect of a factor on human reaction time. It's also useful for Combined Science.
The ruler drop test
AQA's method uses a metre rule, a chair and a table, with pupils in pairs.
- The catcher sits upright, resting the forearm of their dominant arm on the table with the hand over the edge.
- The dropper holds the ruler vertically, with the 0 cm end between the catcher's thumb and first finger.
- The dropper tells the catcher to get ready, then lets go at an unpredictable moment. The catcher catches it as quickly as possible.
- Record the number on the ruler level with the top of the catcher's thumb.
- Rest briefly, then repeat several times. Swap roles.
- Make the change you're investigating, then repeat the whole test.
Turning distance into time
The distance isn't the reaction time. AQA's handbook includes a conversion table: 10 cm is about 0.14 s, 30 cm about 0.25 s and 50 cm about 0.32 s. Notice that three times the distance is not three times the time. The ruler speeds up as it falls, so it covers more distance in each extra hundredth of a second. In AQA's own trial, times ranged from about 0.20 s to 0.31 s.
Variables and repeats
- Independent variable: the factor you change, such as practice, a distraction, or a caffeine drink.
- Dependent variable: reaction time, from the ruler distance.
- Control variables: the same hand, ruler, posture, starting position and people, and the same rest between attempts.
Single drops vary a lot, so take several and calculate a mean. AQA's trial was actually about practice: one person's distance fell from 48 cm to 19 cm over ten attempts. That matters for any other factor. If you test "before" and then "after", the "after" results may be shorter just because of practice, so do some practice drops first.
Ethics and safety
This practical uses people, so it needs their agreement. No one should be pressured to take part, anyone can stop at any time, and personal results shouldn't be shared without permission. Caffeine is a stimulant, but some people avoid it for health reasons, so only test it with your teacher's approval and never make anyone have it. Practice and distraction are simpler factors to test.
Reflexes are not reaction times
A reflex action is automatic and rapid and does not involve the conscious part of the brain. It follows a reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector, with synapses between neurones. Catching a ruler is different. The stimulus is detected by the eye, and you decide to respond, so the brain coordinates it. That conscious step is why reaction times are slower than reflexes.
Common exam mistakes
- Calling the ruler catch a reflex. It involves the conscious brain.
- Treating distance as proportional to time. Use the conversion table.
- Giving "the ruler" as a control variable without saying what about it. Say "the same ruler" or "the same starting position".
- Forgetting practice. It can change results on its own.
- Leaving out ethics. Mention agreement and the right to stop.
Frequently asked questions
How do you work out reaction time from a ruler drop?
Read the distance in cm where the ruler was caught, then look it up in a conversion table to get the time in seconds.
What is the independent variable in the reaction time practical?
The factor you choose to change, such as practice or a distraction. The dependent variable is reaction time.
Is catching a ruler a reflex?
No. Reflexes don't involve the conscious brain, but catching the ruler does, which is why it takes longer.
Why should you repeat the ruler drop test?
Single results vary a lot. A mean of several repeats reduces the effect of random error and makes anomalies easier to spot.
Try it
Say each answer before you flip the card. Then put yourself in Bessel's place: you've found that observers differ from one another by fairly consistent amounts. What would you have tested next?
Braineos is not affiliated with or endorsed by AQA.