Mean machines

9th September 2005 at 01:00
Bob Kibble shows how building siege engines can launch a range of fascinating cross-curricular activities for key stage 3

Trebuchets are making a comeback - if only through television's desire to re-enact history and the cinema's preference for gory battle scenes. The trebuchet is only one of a family of siege engines that were used in anger from about ad 900-1500. Others include the ballista (a form of giant cross-bow) and mangonel (a form of giant sprung catapault). The emergence of gunpowder and cannon marked their demise.

A simple web search under any of these key words will reveal an international subculture of siege-engine fanatics who not only build replica machines, but enter them in competitions.

The use of such devices in the science classroom not only provides opportunities for investigative work, but will open up links between science, materials, history, maths and technology.

By bending the ends of a wire coathanger and adding three rubber bands and a plastic spoon you can make a table-top mangonel-style catapault.

The lower elastic band anchors the spoon handle. The top band provides a stop. The middle band applies most of the force to the spoon. For missiles, I have used grapes and small grape-sized expanded polystyrene balls. By adjusting the position of the bands, the bent angle of the wire frame and the mass of the projectile you can vary the range of the catapault. The coathanger catapault thereby lends itself to a science investigation with at least three variables to be explored.

I use a yoghurt pot as a target, giving students 100 points for a direct "in the pot" hit, 50 points for hitting the outside of the pot, and 20 points for missing it, but landing on the A4 paper target. Advanced groups can be encouraged to calibrate their catapaults before you reveal a target distance for them to aim at.

Key stage 3 curriculum links include:

* Sc1 scientific enquiry, including investigative skills, especially planning and obtaining evidence.

* Sc4 physical processes, including force and motion, force and rotation, moments, air resistance, mass and weight, energy transfer.

* Design and technology, evaluating processes and products 3a, 3b, 3c, including the process of need, design, testing and evaluation. What if you need to move your trebuchet in a hurry? What if the ground is soggy? Would wheels make any difference to the range?

In a science investigation, the planning phase might be supported by offering instructions. However, once made, learners can explore the effects of changing one variable at a time. Do longer spoons reach more distant targets? What effect does moving the middle elastic band have on the range? What is the variation in range over a set of 20 launches? Which one travels further, a grape or a baby tomato?

For more ambitious DIY types, who have a Workmate and saw, a trip to a DIY store will yield all the equipment you need to make a simple siege engine.

I made my first wooden trebuchet from photos and instructions found on the web (see left). It should be noted, however, that the flying arm, which the sling is attached to, is a potential hazard. Moving the sling release pin, located at the tip of the arm, could easily result in an accident for unsuspecting users or for those who think the model is a toy.

For a target I use toy wooden bricks. During teachers' workshops, typically a group of teachers will build a castle wall, while others set up the attacking siege engine. We fire approximately 50 gram balls of Plasticine which have a range of three to six metres. What effect does changing the mass of the Plasticine have? If this seems a shade ambitious, then dig out some coathangers, elastic bands and a bunch of grapes, and don't look back.

* For more details about trebuchets log on to

Bob Kibble lectures in science education at the University of Edinburgh

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