Hi! So this is a more general question about a question that I have already solved but I'm still confused and it seems like I'm missing the concept here. 1. The problem statement, all variables and given/known data I'll try to recall the question as best I can from memory. A person is standing at a certain height h and shoots an object with some mass m at some angle z with a slingshot that acts as a spring with a k of k. The question asked for the velocity after the slingshot was released. 2. Relevant equations Kf + Ugf = Ki + Ugi 1/2mvf^2 + mgy = 1/2mvi^2 + mgh Ws = 1/2 kxi^2 + 1/2 kxf^2 3. The attempt at a solution So basically what I did was plug in the values to find the work of the spring on the ball which I found to be 1/2kd^2. My main question is whether or not it is correct to substitute this value into the first equation above for Ki, the initial kinetic energy. This provided the correct answer and I found velocity by solving for Vf. I'm not sure if this makes any sense because I'm pretty confused myself. Thanks for any assistance.