What it shows
A dropped ruler measures how fast your nerves and muscles respond. How far the ruler falls before you catch it gives your reaction time, and you can test how practice or a second task changes it.
The science
When you see the ruler move, the signal goes from the eye to the brain, then along motor nerves to the muscles of the hand. This takes a short but measurable time. A ruler falling from rest travels a distance d = ½ g t², so the reaction time t = √(2d/g), with g = 9.8 m/s². A fall of 20 cm means about 0.2 s. The test measures voluntary reaction, and practice or a second job for the brain can change it.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 30 cm ruler
- A partner
- A table and chair
- A notebook and a calculator
- A phone calculator or a printed table of distance and time
- A few easy questions or a counting task for distraction
Safety
For this project: Sit down during the test and keep the ruler soft and light. An adult should supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Ask your partner to hold the ruler at the 30 cm end, so the zero end hangs down between your open thumb and finger.
Place your finger and thumb at the zero mark with the ruler not touching, and look at the ruler.
Your partner drops the ruler without warning, and you catch it as fast as you can.
Read the centimetre mark at the top of your thumb as the distance fallen and write it down.
Repeat 10 times and write the distances in a table, then calculate the time using t = √(2d/9.8), with d in metres.
Now repeat 10 trials with your other hand and compare the average times.
Repeat 10 trials while you also count backwards from 100 by sevens, so your attention is split.
Plot a graph of reaction time against trial number to see if practice improves it, and compare with the distracted case.
What you should see
Typical times are 0.15 to 0.25 s. Practice may lower the time slightly over trials, and the distraction task usually makes it longer. The average for each condition is written in your table.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the formula that links distance and time?
d = ½ g t², so t = √(2d/g).
2What path does the signal follow in a reaction?
Eye to brain through sensory nerves, then from brain to muscles through motor nerves.
3Why drop the ruler without warning?
So the person cannot guess the moment and cheat the time.
4Why does distraction slow the response?
The brain handles two jobs at once and the decision takes longer.
Take it further
Compare reaction times of the same people when they are fresh in the morning and when tired in the evening.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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