What it shows
A flat sheet of paper bends easily, but the same paper rolled into a tube becomes strong. The bridge shows how shape, not material, decides strength.
The science
When a beam bends, the top edge is squeezed and the bottom edge is stretched. A rolled tube places its material far from the centre line, so it resists bending much better than a flat sheet. This is why bicycle frames and scaffolding use tubes. Triangles in a truss add stiffness because they cannot change shape without changing the length of a side.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- About 20 sheets of old newspaper
- Masking tape or flour paste
- Two stacks of books as piers
- A small bucket with a string handle and a bag of pebbles as load
- A kitchen scale or the printed weight of the pebbles
- A ruler and a pencil
Safety
For this project: Keep fingers and feet clear when loads are added, and put a cushion or a folded cloth under the bucket in case the bridge fails. An adult must be present if heavy loads are used.
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.
Step 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.
Roll one sheet of newspaper tightly around a pencil diagonally and tape it to make a tube; make at least 15 tubes.
Place two book stacks 30 cm apart and test a flat folded sheet as a bridge by hanging the small bucket from its middle with the string and adding coins or pebbles.
Lay three tubes side by side taped together as a beam and place it across the books.
Hang the bucket from the middle and add pebbles one by one until the bridge sags or fails.
Record the load in grams that it held.
Build a second bridge using the tubes in triangles to form a truss and test it the same way.
Compare the loads of the flat sheet, the tube beam and the truss on a bar chart.
Write what changed between bridges: the shape, not the amount of paper.
What you should see
The flat sheet fails with a few coins, the tube beam holds hundreds of grams, and the truss usually holds more still.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why is a tube stronger than a flat sheet?
Its material is farther from the centre so it resists bending better.
02Why are triangles used in bridges?
A triangle cannot change shape without a side changing length.
03What force squeezes the top of a bridge beam?
Compression.
04What force stretches the bottom?
Tension.
Take it further
Make tubes of different thickness and plot the failure load against tube diameter.
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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