<Mentor's note: moved from a general forum and therefore no template> This is the equation of potential energy in rubber bands, however, I don't know how to calculate the inertia of the arm of catapults or get the k constant. And what does n mean? Basically, i don't understand the whole equation. The energy balance, where N is the number of rubber bands, k is the spring constant of a single rubber band, x0 is the unstretched length of a rubber band, m the mass of the ball, and I is the moment of inertia of the arm. Can someone please show me an example of how to calculate this? I used a marshmallow which is 0.007 kg, I stretched the catapults to 0.1 m, it has an angle of 35 degrees range=0.793 initial velocity=2.88 m/s time=0.337 s max height=0.139 m horizontal velocity=2.36 m/s vertical velocity=1.65 m/s Thats all i know and i hope it helps! please help me figure this out!