The Trebuchet

16 November 2008

Ingredients

Some cardboard Some cardboard A biro A biro or pencil
A pen lid A pen lid or something of similar weight that you can attach string to Some wire A thin piece of wire or a plastic bag tie
String Some string A mug A mug

Instructions

A trebuchet works a bit like a big see-saw and the first job is to make the arm of the see-saw. First of all, cut a coffin-shaped piece of corrugated card (about 20cm by 6-7cm) with the corrugations running lengthwise. Then fold the carboard twice lengthwise either side of the centre, leaving 2-3 corrugations between your folds.

Carboard cut to shape Folded card
Cut a coffin-shaped piece of corrugated card Make two lengthwise folds either side of the centre line.

Now push the biro through the both sides of the card arm, about a quarter of the way along from the widest end.

Make 2 small holes in the cardboard on the centreline, one at each end. At the narrow end push the wire tie half-way through the hole and bend it over on either end.

Result

You should find that after a few adjustments the trebuchet can throw its projectile a surprising distance.

Explanation

A trebuchet basically works like a see-saw. If you have one heavy person on a see-saw and one light person on the other end the light person will move upwards. If you put a very heavy person close to the pivot on one end and a very light person on the other end, the light person will move up much farther than the heavy person moves down. This is because the light person's side of the see-saw is so much longer. If the see-saw rotates they will have to move a lot farther. If you replaced the heavy person with several tonnes of rock it is going to fall towards the ground quite quickly so the light person is going to be launched upwards several times faster than the rocks are falling.

A see-saw Offset see saw
If one person is heavier than the other on a see-saw they will go down and the lighter person will go up If the heavy person sits near the pivot and the light person a long way away, the light person will move up much faster than the heavy person falls down.

A trebuchet basically works in this way, with a slight complication of the sling. This starts to rotate around the arm as well as moving with the arm. This means that as it releases the projectile is both moving with the arm and rotating around it, so it can end up moving at up to twice the speed of the arm. This greatly increases the range of the trebuchet at the cost of a piece of rope.

The loaded trebuchet Trebuchet shooting The sling releases
The trebuchet starts with lots of potential energy in the weight The weight falls, lifting the other end of the arm. The projectile moves with the arm and starts to spin around it. The spinning sling and the moving arm add together to make the projectile move very rapidly and hurtle towards a castle wall.

Real trebuchets would normally hurl a 100kg stone 300m but some of them were built to launch 300kg or even 800kg rocks several hundred metres. And because you can repeat the same set-up very accurately, by starting the projectile in the same place every time and using the same counterweight, you could keep hitting the same piece of wall again and again until it collapses.

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