What it shows
A ruler pressed on a table and plucked gives a higher note when less of it hangs over the edge. By counting vibrations per second for each overhang length, you find how length controls frequency.
The science
A short, stiff strip vibrates faster than a long, floppy one. The frequency of the vibration rises when the free length is reduced, because the bending force on a short strip is greater compared with its mass. The sound is higher in pitch when the frequency is higher.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A flat steel or plastic ruler
- A table with a straight edge
- A phone with a slow motion video or a tuner app
- A metre rule
- A notebook
- A friend to count
- A graph paper
Safety
For this project: Keep faces away from the free end of the ruler and avoid hitting hands. Do not use a ruler with sharp edges.
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
7 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.
Press the ruler firmly on the table with its end hanging over by 25 cm.
Pluck the free end by pushing it down and releasing it, and listen to the note.
Record a slow-motion video of the ruler, or count the number of back and forth movements in two seconds.
Calculate vibrations per second by dividing by two.
Shorten the overhang to 20, 15, 10 and 5 cm and repeat.
Write the length and the frequency in a table.
Draw a graph of frequency against length and describe the trend.
What you should see
As the overhang gets shorter the vibration gets faster and the note rises. The graph slopes downward, showing that frequency falls as length grows.
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 frequency?
The number of vibrations per second.
2What happens to pitch when the length is shortened?
It rises.
3Why can you hear it?
The ruler pushes the air and makes sound waves.
4What is the unit of frequency?
Hertz.
Take it further
Compare a steel ruler with a plastic one at the same overhang and explain the difference.
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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