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How do you make string stand on its end? To find out, read on…

Safety: This activity involves cutting a plastic bottle up. Ask an adult for help.

You will need

• String
• Good scissors
• 1.25 L soft drink bottle. You want one with a big label, so you have a nice big cylinder of plastic to work with.

What to do

1. Empty the bottle.
2. Carefully squish the bottle and then cut into it with your scissors just above the label.
3. Cut the around the bottle, removing the top section.
4. Cut straight down the side to the area below the label, cut around the bottle again, removing the bottom section. You will be left with a rectangle of plastic covered by the label.
5. Cut several strips of plastic, around 2 centimetres wide and 10 centimetres long, running up and down the bottle.
6. Cut a small 5 millimetre triangle into the ends (shortest sides) of each strip.
7. Cut a piece of string about 50 centimetres long.
8. Tie a knot every 1.5 centimetres along the string.
9. Time to get building! Grab a strip of plastic, and jam the first knot of the string into the notch on one end of the strip. Bend the strip and then stick the fourth knot into the notch on the other end of the strip.
10. Check the string. The plastic should be pulling it tight. If it is loose, try turning the strip over and then trying again.
11. Take a second strip of plastic. Put one notch on the third knot, and the other notch on the sixth knot.
12. For the third strip, hook it between the fifth knot and the eighth knot.
13. Hold one end of the string and let it hang. Adjust the pieces of plastic so they are evenly spaced.
14. Without letting go of the string, use your other hand to hold the string between the bottom pieces of plastic.
15. Let go with your top hand. The string will remain standing straight up!
16. Add more pieces of plastic by following the same pattern. How high can you make your tower?

What’s happening?

String isn’t normally thought of as being strong. Certainly, it’s not what most people would use to make towers out of. But it turns out that string isn’t always weak.

There are lots of different ways to measure strength. Two important ones are compressive (squeezing) strength and tensile (stretching) strength. Just because a material is strong in one way doesn’t mean it is strong in the other.

A piece of string has almost no compressive strength. It’s just too bendy. But if you pull on it, you’ll find it is surprisingly strong. It has lots of tensile strength.

To exploit this strength, we need to provide a pulling force along the string. In this activity, the plastic pieces act as springs, stretching the string and allowing it to be strong. If the tower gets too heavy, it will overload the springs and the string will lose its strength.

In the real world, engineers often combine two materials with different strengths. For example, pre-stressed concrete has steel cables running through it. The cables are stretched by the concrete, making them strong. In return, the concrete is squashed by the cables, making it stronger too!

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