Mechanical Engineering
How do catapults work?
Catapults, used a lot in the middle-ages as a weapon of war, are really a very large lever. There were a few common designs, but you can think of this as a very long pole which would have an axil (a rod) running through it which was much closer to one end than the other. This axil would be parrellel to the ground and sitting on either a post with a "Y" or in a larger structure with a hole in it. The axil then was free to rotate in this, essentially this was a bearing (similar to what you would see on a wheel's axil). The item that was to be thrown would be placed at the end farthest from the axil, either held with a rope which was hanging on the pole (in a notch cut in the pole) or even in a basket. The pole would be locked down with a rope and the short end would either have weights attached to it directly, or quite often have a rope attached which would be connected to weights, possibly on a pulley system. After the weights and projectile were loaded, and of course the catapult was aimed towards the target, the rope locking the pole in place would be cut - now you have a lever-arm, it would swing around and throw the projectile -- often to very great heights and distances. Catapults were often used in "Storming a Castle" or getting items over a city wall (cities-states in those days were often surrounded by walls for protection). The weapon could be a number of things, like burning bundles of hay intending to set the city on fire. Sometimes, catapults would use biological weapons, by taking the rotting carcasses of dead animals (like horses) and shooting them over the walls with the intention of spreading disease within the city.