Bridges intro

Before trying to explain how bridges work it is important to look at the properties of different materials and try to define the term ‘strength’ of a material or structure.

Paper and card can be used in all sorts if ways to help introduce the bridges topic. Below are some ideas for demonstrations or class activities which are easy to do and full of good science and technology.

How strong is a piece of paper?

Try holding a single sheet of paper by a short edge and then hold it out horizontally in mid air. The sheet of paper usually bends and flops down. It is not strong enough to resist the effect of gravity pulling down on it and it bends under its own weight.

Try placing a sheet of paper longways across a gap between two piles of books or resting on the backs of two chairs place a short distance apart. Again the paper bends and may collapse into the gap.

These simple demo’s suggest that paper is a very weak material because it cannot support its own weight and bends very easily.

However! There are other ways we can test the strength of paper. We could try stretching it instead of putting it in a situation where we are allowing it to bend.

If we hold near one end of a piece of paper with both hands at the sides (fingers below and thumbs above gripping tightly but not crumpling the paper) we can have a ‘tug of war’ with another person on the opposite end of the sheet! Take care not to pull too hard or the paper will tear and you may fall back!!!

The paper usually lasts long enough in the tug of war to make the point that paper is quite strong when we test it by trying to stretch it!

So! The original question really wasn’t a very good question at all because paper is weak when we try and bend it but strong when we try and stretch it!

The conclusion is that the term ‘strength’ requires a proper definition. We must be clear about the way in which a FORCE is applied to a material before we can learn anything about the strength of the material.

A paper sheet is poor at resisting a BENDING FORCE but very good at resisting a STRETCHING FORCE.


Paper can be made into different shapes. Can this affect the strength?

Make a paper cylinder by curling a sheet of A4 paper around short side to short side so that they overlap a little then add two small pieces of tape to hold in place.

The paper cylinder can stand up for itself! This indicates an increase in strength and it can be seen that the cylinder stands upright and doesn’t flop about - of course the tape is helping with this!

How can we find out how strong it is?

We could add objects on top and see how much weight it can stand before it collapses. The easiest objects to add are small thin reading books. They can be added one at a time. Take care to place them on top in line with each other nice and evenly and count how many before it collapses.

It’s quite amazing how much weight a cylinder made from a single sheet of paper can support. The weight it carries is hundreds of times more than its own weight!

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