1. The problem statement, all variables and given/known data So I have been building a mouse trap car at my school, and I need to hand in a report on it tomorrow. I don't have all of the measurments at the moment, but the only thing I want to know is how to calculate this. I want to see how much energy is lost to friction/warmth/sound by dividing the total kinetic energy(I have figured that out!) by the potential energy. The mouse trap is 180° or π rad from the closed position when it's set. So =π rad I only have the average speed. 2. Relevant equations Potential energy in a torsion spring: Hooke's law: Work: W=F*s Torque: 3. The attempt at a solution So at first I was thinking that that k must be equal to T / , and that T must be equal to the arm length times the average force used to set the mouse trap. k should therefore be k=T/π=(r*π*F)/π. Using the torsion spring equation, I then get U=(r*F*π)/2. I then thought that the work in order to set the mouse trap should be equal to the potential energy. I used the circumference of a semicircle as the stretch. I then got W=F*π*r, but this is twice as much as I got with my other ""solution"". I then thought that I might be able to just use that the X in Hooke's law is equal to the circumference of the semicircle. k should therefore be F/(π*r). Using the first equation, I then get U=(1/2) * (F/(π*r)) * π^2= (F*π)/(r*2) I'm also not sure about how F should be measured. Should I take the force used when you're barely lifting the mouse trap + the force used when it's fully set and divide that by 2? Sorry if anything was unclear! What do you reckon I should do?