Trebuchets, Levers and Mechanical Advantage

Trebuchets have been around since about the 4th century BC, when Chinese soldiers used their muscles to power mounted slings. But the trebuchet heyday didn't arrive until the Middle Ages, after people figured out a counterweight could do a lot of the work for them. In medieval siege warfare, gravity-powered trebuchets launched everything from giant stone balls to the decapitated heads of enemies.

The Long Arm of Leverage

A trebuchet uses same basic principle as a claw hammer: leverage. The hammer's handle is the long arm of a lever. Its head is the fulcrum, and the claw is the lever's short end. Braced under the head of an errant nail, the claw is ready to do some work.

When you push down on the handle of the hammer, you don't have to exert yourself too much, but you move the handle a long way. On the other side of the fulcrum, the claw doesn't move very far, but because energy can't be created or destroyed, it does a lot of work in that tiny motion. If all goes well, you can yank the nail out of the wall. It's all a matter of proportion -- your energy input has to match the claw's energy output. This ratio of input to output is called mechanical advantage.

Storming the Castle ... with a Sling

In a trebuchet, the lever is a beam on an off-center pivot. The long end of the beam holds a sling, which holds the projectile. The short end supports a heavy counterweight. When the trebuchet is cocked and ready to go, the counterweight is in the air and full of potential energy. To fire the trebuchet, all you have to do is let the counterweight fall. It moves a short distance toward the ground while the sling on the long end flies toward the sky. The sling has a long way to travel in a short amount of time, so it moves quickly, bestowing the projectile with enough kinetic energy to knock down walls.

The Most Bang from Your Sling

Because of mechanical advantage, you can launch a projectile farther with a heavier counterweight, but a gigantic counterweight can be impossible for people to manage. That's why medieval military engineers found other ways to make trebuchets more efficient. One method was to put the counterweight on a hinge. During the descent, the weight would follow the motion of the beam and swing a little, adding some extra power. Another option was to prop the counterweight up before launch so it could fall a little bit farther. Not all of the enhancements to the typical trebuchet focused on the counterweight, though. A trench under the device also made room for a considerably longer sling. This essentially made the long arm even longer, adding more windup before the pitch.

So how effective were these medieval siege engines? One example is a trebuchet known as Warwolf, built for King Edward I's siege of Stirling Castle in 1304. Warwolf reportedly hurled a 300-pound (136 kilogram) ball with enough force and accuracy to destroy a large section of the castle wall. So it's no surprise that when faced with a trebuchet, many siege defenders made a prudent choice: surrender.