Hi, I have a big problem.... I got a project on building a catapult. Easy, right? WRONG! We are supposed to determine the arm length without trial and error and I have no clue what to do. Please please help! Catapult is supposed to be built with 3/4" paper clamp as the only energy source and it must project a 2gram pingpong ball 1.5m. The catapult is a classic 2nd class lever. How would I calculate the arm length?