What it shows
A four-sided frame can collapse but a triangle cannot change shape without changing the length of a side. A dome of triangles is strong and light.
The science
A triangle is the only polygon that is rigid when its sides are fixed. A square or a rectangle can be squashed into a rhombus, but a triangle cannot. Engineers brace frames with diagonals to make them stiff. A geodesic dome is built from triangles that spread a load across the whole shell, so it can span a large area with very little material.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- About 40 drinking straws
- A packet of pipe cleaners or thread
- Scissors (adult help)
- A ruler
- A square of straws for comparison
- A tray
Safety
For this project: An adult must cut the straws. Cut pipe cleaner ends are sharp, so bend them in.
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.
Cut 30 straws to the same length, say 10 cm, and 10 shorter ones.
Make a triangle and a square using pipe cleaners as joints and press both sideways.
Add one diagonal straw to the square and press again.
Build a pentagon from five long straws and then fill it with triangles, a pentagon pyramid.
Join triangles into a half-dome of about 30 straws.
Place a small book on top of the dome and see if it holds.
Do the same on a dome without diagonals and note what happens.
Count the triangles, vertices and edges and write what you find.
What you should see
The square sags but the braced one holds; the dome carries a light book.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a triangle rigid?
Its shape is fixed by its three side lengths.
2How can you stiffen a square?
Add a diagonal.
3Where are domes used?
In stadiums, planetariums and old monuments.
4What does Euler’s formula say?
Vertices minus edges plus faces equals 2.
Take it further
Check Euler’s formula V minus E plus F equals 2 for your dome.
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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