1. The problem statement, all variables and given/known data A ball with mass 2kg is travelling 2m/s. It impacts and sticks to a hanging, stationary 4kg pendulum. How high will the pendulum + ball go? 2. Relevant equations KE = 1/2mv2 PEg = mgh mava = m(a+b)v(a+b) 3. The attempt at a solution I tried to solve this question assuming conservation of energy in the collision, while a friend tried to solve it using conservation of momentum, and we got two separate answers. The question itself dos not say whether the collision is elastic or inelastic (is there some way to figure it out from the given information?). My attempt: Let the ball be represented by a Let the ball + pendulum be represented by (a+b) KEa = PEga+b 1/2mava2 = m(a+b)gh 1/2(2)(2)2 = 6gh 4 / 6g = h 0.066 = h My friend's attempt: mava = m(a+b)v(a+b) 2(2) = 6V(a+b) V(a+b) = 2/3 KE(a+b) = PEg(a+b) 1/2m(a+b)v(a+b)2 = 6gh(a+b) 1/2(6)(2/3)2 = 6gh 1.33 = 6gh 1.33/6g = h 0.022 = h Neither of us can see where the other went wrong; can someone please shed some light on which way is correct?