I just took a physics test today and there was one problem that I cannot figure out if I did correctly. The problem involves a catapult pictured here: http://i39.tinypic.com/a2t76h.jpg The bar is 2m long and has a mass of 3kg. The rock at the end of it has a mass of 5kg. The spring is .3m from the pivot. As shown, when the spring is unstretched/uncompressed the bar makes an angle of 30° above the horizontal. It is then pushed down so it now makes an angle of 30° below the horizontal. My first question is what is the x for the spring when determining the elastic potential energy? Because I just set up triangles and figured it out to be .3m but my friend used the relationship between linear and angular distance, d=θr, and got x to be larger. But I believe that gives the arc length traveled by the end of the spring as it is compressed rather than how much it is actually compressed but I could be wrong. Here is a picture of what I think it is like: http://i44.tinypic.com/5zm6om.jpg And so after you figure out what x is, what the problem asks for is the velocity of the rock as it passes back through the original position? (Using conservation of energy) So how would this be done and what is the answer? Because I ended up with a negative value while trying to find the answer meaning the spring did not have enough energy to get the catapult back to the starting position but this is probably wrong because that would be a dumb test question. Sorry this post is so long but please help!!!! I've been going nuts trying to figure out what I did wrong!!!