Helped my student make this catapult for a high school physics project. Used 2" scrap lumber, screws, 2 cup hooks, and a short bungee cord. Made a projectile seat out of card stock and hot glued it on. Also copied a protractor and taped it to the side for calibration. Throws a small bean bag about 10 meters (32.8 ft) at full force.
Atomic Shrimp’s Two Hour Trebuchet was designed and constructed in a mere 2-hours (hence the name). Amazingly enough, there were no blueprints used for the build, which uses 1-ich batten for the frame and throwing arm. A heavy engineering brick strapped to a wooden skid serves as the counter weight, which is used to fire tennis balls as projectiles. (Link)