A catapult is a mechanical device designed to launch a projectile over a distance. It relies on basic mechanical principles, primarily classifying it as a Lever. The lever is one of the six classic simple machines, which are fundamental devices that change the direction or magnitude of a force.
The Catapult as a Lever System
A lever is defined by three main components: the fulcrum, the effort, and the load. The fulcrum is the fixed point around which the lever arm pivots, while the effort is the input force applied to the arm. The load is the object being moved or resisted, which in a catapult is the projectile.
The catapult’s rigid throwing arm acts as the lever, and the axle or hinge connecting the arm to the base serves as the fulcrum. The primary function of the catapult is to transform a relatively slow input force into a very high velocity for the projectile. This is achieved by creating a long “load arm” distance, which is the segment of the lever arm between the fulcrum and the projectile.
By applying the effort over a short distance, the lever converts that movement into a much larger arc traveled by the load. This design trades mechanical advantage, which would normally reduce the needed input force, for a significant speed gain at the end of the arm. The projectile’s final velocity is directly related to the length of this load arm and the speed of the lever’s rotation.
Applying the Three Classes of Levers
While all catapults operate on the lever principle, their specific mechanical design dictates which of the three lever classes they represent. This distinction depends on the relative position of the fulcrum, effort, and load along the arm.
The Trebuchet, a well-known gravity-powered siege engine, typically functions as a Class 1 lever where the fulcrum is located between the effort and the load. In this design, the counterweight (effort) falls downward on one side, rotating the arm around the pivot point (fulcrum) and launching the projectile (load) on the opposite side.
Other designs, such as the Onager or a torsion-powered Mangonel, often operate as a Class 3 lever. This class is characterized by the effort being situated between the fulcrum and the load. The fulcrum is at the axle, the projectile is at the end of the arm, and the force from the twisted ropes or springs (effort) is applied to the arm somewhere in the middle. This Class 3 arrangement maximizes the output distance and speed of the projectile.