Building a catapult serves as an excellent class activity for lessons on projectile motion or medieval warfare. An engaging project, catapult building requires inexpensive construction materials -- such as wood or PVC -- and encourages as much innovation as possible in the designs. A catapult project challenges students to fulfill three main objectives: longer range, higher projectile weight capacity, and greater accuracy. Students are free to choose among several types of catapults which they can modify for optimum performance.
The choice speed weapon in medieval Europe, the trebuchet -- also called the "Ingenium" -- survived up to a century after the invention of gunpowder. A trebuchet has four main parts: the frame, the beam, the counterweight, and the sling. It harnesses the potential energy of the suspended counterweight and uses that to hurl the projectile. Ideally, the counterweight weighs as much as 80 to 100 times the weight of the missile. Trebuchets are an excellent choice for catapult projects because of the straightforward scaling involved -- areas as the square of the lengths, and volumes and weights as cubes.
Ballistas were the giant crossbows of the Greeks, which propelled massive darts or arrows along a straight trajectory using the torsion in two twisted skeins of rope, hair, or sinew. Because they have more moving parts than the trebuchet, model ballistas are slightly harder to scale, design, and build. Braided nylon is an excellent material for the cord bundle. The strength of this bundle will determine the ballista's missile weight capacity. Provided the frame is strong enough and the cords sufficiently twisted, a projectile can cover a long range with superb accuracy.
A mangonel, whose name comes from the Latin word "manganon" meaning "engine of war", is in essence a torsion catapult. Like the ballista, a sturdy twisted skein provides the driving force for its missiles. It has a throwing arm similar to that of the trebuchet, but instead of a sling, it uses a bowl- or spoon-shaped extension. It loses to the ballista in accuracy and to the trebuchet in the size -- though not necessarily the weight -- of the projectile it can throw. However, it can cover a longer range than the trebuchet and achieve faster missile speed. Students usually find mangonels the easiest to build.
Many consider da Vinci's catapult a vast improvement on the catapult designs of his age, although no records exist of their being built or used in battle. The design relies on a simple leaf spring for the energy needed to throw the projectile. This catapult uses a throwing arm similar to the mangonel, but longer and attached to a rotating drum at one end. Cocking the throwing arm causes the accumulation of energy in the leaf spring. When the throwing arm is released, this energy rotates the drum and gives the arm a huge amount of acceleration. This design is efficient and excellent for illustrating energy transfer among a machine's components.
