To run the programs, unzip the files and run the Xcode project. If you click Run, the project should run on the device / simulator.
I made a basic slingshot. I mapped a football texture to use texture mapping. I made an Am chord play when the balls hit the walls and play notes of the Am scale depending on the velocity of the colliding balls. I restricted the number of balls to 10 - when you try to add more it goes back to 0. I left the rubber band in there so that you can add multiple balls from the same place.