I need to explain why, in our tests, the efficiency of our slingshot device remained constant when the angle of launch was changed and the extension (how far the rubber band was pulled back) was constant. Basically it was a 1m long wooden plank held up on bricks, with a rubber band attached to 2 nails at the top of the board. We put a rubber ball in the rubber band and shot it from different angles. We calculate efficiency in terms of energy lost. Thanks!