I'm a high schooler and female... give me a break. this catapult is pretty bad... but working in school with limited supplies is very difficult. :) I worked in a group with 2 others, and we attempted to build a catapult.
Step 1: Collected Supplies
We used rebar, 7 2"x4" wood planks, a metal pole, a mental stake (of extremely large proportion), 2 springs, a tin can, a hook, some braces to hold the wood together, and a whole lot of nails :)
Step 2: Basic Foundation
We screwed 4 wooden boards together and put two standing up to hold the arm (seen in the picture). We drilled holes for the metal rod to go through near the top of the vertical boards, and near the bottom for the thick stake to go through. We used the stake to hold the springs to the arm, where they slid on to the bar to attach and attached through holes on the arm, and we used human force to bend the strings. We used such a thick and rugged bar because the inertia from the springs is so great we had to use something that wouldn't bend or bow under pressure.
Step 3: Adding the Arm
We screwed a tin can to one end of a metal pole, drilled holes big enough for the rebar to enter through the middle of the pole, and 2 small holes near the bottom of the arm for the springs to be attached to.
Step 4: Gettin' It Done
We applied another wooden board to stop the arm from having too large of a displacement from its beginning point, which would render the arm and force the ball a farther distance, rather than the arm launching the ball straight into the ground. The law of gravity takes affect on the ball, and even though the ball does not have much weight, we still need it to have the highest possible acceleration. We set the stopping board up so that the arm would launch the ball at about a 95 degree angle, hoping the projectile would gain maximum speed and velocity.
Step 5: Results
Newton's Second Law shows that the ball will fly as the arm flies, but have as much acceleration as the hit on the stopping board. The graph shows how the ball should have flown 76 meters per second. With our final test, the ball flew 22.8 meters before landing on the ground. Not the best catapult in the class, but it worked. Good enough for me :)