Catapults operate using stored energy to hurl projectiles great distances. Both catapults and ballistas operate by storing tension, in either twisted rope or in pieces of wood that have been flexed. The trebuchet, a similar device, makes use of a counterweight to transfer the force needed to launch a projectile.
Catapults were ancient siege engines that used a simple principle of physics, known as stored energy, to hurl a payload without the need for explosives. The application of sufficient force to an object can change its shape. Elastic objects, such as wood or rope, are able to store potential energy when force is used to change their shape. When this stored potential energy is later released, it is transferred to the projectile or payload being launched.
Trebuchets are siege engines very similar to catapults and operate using the same principles of storing potential energy. While catapults store potential energy in an elastic substance, trebuchets make use of a pivoting beam and counterweight-system that is able to store kinetic energy when it is lifted into position. Both catapults and trebuchets make use of gears and winces that transfer direct force, making it possible to store and build potential energy through repeated applications over a long period of time.