We typically think of a catapult as something that was used in the Middle Ages to destroy the walls of a castle as in the poster shown here. But Catapults have a very long history dating long before the time of castles and they were developed and designed in many different ways by many different cultures over the centuries.
What is a catapult? The general definition is that a catapult is a machine that stores energy then quickly releases the energy to fire a projectile. To be a catapult the machine generally has to be too large for a person to carry. If we just used the storage and release of energy to define a catapult then a longbow would also fit this definition. So the size of the machine is important.
How did catapults develop? Catapults are an offshoot of the Crossbow. Over time crossbows got larger and larger. The went from being a hand-held weapon to something called the Belly-Bow which were so large they had to be braced against a knight's belly. From there they got even larger and became something called a stand crossbow where they were mounted on stands. Eventually they got large enough to be defined as something in their own right and no longer crossbows. This size change also brought about changes in how they operated.
The earliest catapultWhen did the first catapults appear and where? The earliest writings of catalpults were that they originated in China around the 3rd and 4th Century BC and this type of catapult was much like a big crossbow. They stood around 8 feet tall. The illustration here shows this early type of catapult. You can see by the design that it is a natural extension of the crossbow. It is pretty much a very big crossbow! But the unique development that turns it into what we consider to be a catapult is the swinging arm. This is mounted on a pivot and thus we have a catapult. The crossbow string itself is not used to fire the projectile, it is used to move the arm which hold the projectile.