What it shows
The distance a ruler falls before you catch it tells how fast you react. Comparing hands and signals shows what can speed up or slow down a response, which is important in cricket and sprint starts.
The science
A freely falling object falls a distance d = (1/2) g t squared, so t = square root of (2 d / g). A catch at 10 cm takes about 0.14 s and a catch at 20 cm takes about 0.20 s. The delay comprises the time to see or hear the signal, send nerve messages, and move the muscles.
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 friend to drop it
- A table with an edge
- A bell or clapper for the sound signal
- A notebook
- A calculator
Safety
For this project: Keep to a calm activity and take care that no one is hit by the ruler. This is a fun measure, not a medical test.
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.
Sit with your arm resting on the table and your hand beyond the edge, fingers ready to pinch.
Your friend holds the ruler with its zero mark level with your fingers, and then drops it without warning.
Catch it and read the distance at the top of your thumb.
Do ten drops with the dominant hand and write each distance.
Repeat with the other hand.
Next, repeat with your eyes closed, and your friend says "now" or claps just as the ruler is let go.
Convert the average distance to time using the formula above.
Compare the results in a bar chart of three bars: dominant hand, other hand and sound signal.
What you should see
Reaction times of around 0.15 to 0.25 seconds, usually a little faster with the dominant hand and with practice. A sound signal may give a slightly different time from sight.
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 reaction time?
The time between a signal and the start of the response.
2Why does a longer catch distance mean slower reaction?
The ruler had longer to fall before you caught it.
3Why do sprinters practise the start?
Practice shortens the response to the starter’s signal.
4Why is a blind drop needed?
So that you cannot guess when it is released.
Take it further
Make a reaction time scale from the formula, marking each centimetre with the time in milliseconds.
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.
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