1. The problem statement, all variables and given/known data A stone of mass 5 g is projected with a rubber catapult. If the catapult is streched through a distance of 7 cm by an average force of 70 N, calculate the instantaneous velocity of the stone when released. 2. Relevant equations work done in elastic material w = 1/2 fe = 1/2 ke2 = 1/2 mv2 - the kinetic energy of the moving stone. (a) 1/2 fe = 1/2 mv2 Or (b) 1/2 ke2 = 1/2 mv2 3. The attempt at a solution mass m = 5g = 0.005kg; extension e = 7cm = 0.07m; force f = 70 N; velocity = ?; using: 1/2 fe = 1/2 mv2 v = square root ( fe/m) v = (70 * 0.07/0.005) = 31. 304 m/s Using either of the equations gives the same result. Am not sure wether I have arrived because the book am using gave a different solution for the problem. So you watching at this thread, what do you think?