With the Competition at the end of the month, I started looking at how trebuchets work. However, first we should look at why they are diffrent from Catapults
Catapults generally used a large, springy piece of wood which would have been wound up. This then give tension to the wood and when released the arm pulls up and hits a stop then releasing the projectile.
Trebuchets, in contrast, use a weight which pull down a lever arm, launching it up into the air. A sling is attached which then released the projectile. Trebuchets are generally capable of hurling great amounts of weight and became much more common in medieval warfare in later years.
A trebuchet consists of five basic parts: the frame, counterweight, beam, sling and guide chute. The frame supports the other components and provides a raised platform from which to drop the counterweight. The counterweight, pulled by gravity alone, rotates the beam. The beam pulls the sling. The guide chute guides the sling through the frame and supports the enclosed projectile until acceleration is sufficient to hold it in the sling. The sling accelerates and holds the projectile until release.
One end of the sling is fixed to the end of the beam, while the other is tied in a loop and slipped over a release pin extending from the end of the beam. As the beam rotates, it pulls the sling, with its enclosed projectile, down the guide chute. As the sling exits the chute, it accelerates in an arc away from the beam, but because the beam is still pulling the sling behind, the loop is held on the pin. The sling continues accelerating through its arc until it eventually swings ahead of the release pin. At this point, known as the release angle, the loop slips off the pin and the sling opens releasing the projectile.
To get deep into the physics, watch this video...