http://imgur.com/Pc0nd Basically, a block the mass 1 is traveling with velocity 1 towards the stationary spring system with mass 2. How much will the spring system compress? I originally thought the answer was C. (highlight) 1/2mv^2=1/2Kx^2 (mv^2)/K=x^2 Then equation C But NO! The answer is D (highlight) For the initial KE, they set the formula to accompany the second mass. m1v1=(m1+m2)vf vf=m1v1/(M1+m2) 1/2(m1+m2)v^2 1/2(m1+m2)(m1v1/(M1+m2))^2 1/2(m1v1)^2 1/2(m1v1)^2=1/2Kx^2 Then derive equation D. Why was is it not my first choice? I have no idea why they decided to incorporate m2 into the KE equation. The mass 1 is the only mass with initial velocity, and 1/2m1v^2 IS THE COMPLETE ENERGY being applied in the system. It does not need mass 2. I have no idea why they decided to solve it the way they did.