1. The problem statement, all variables and given/known data When a mass m sits at rest on a spring, the spring is compressed by a distance d from its undeformed length. Supposed instead that the mass is released from rest when it barely touches the undeformed spring. Find the distance D that the spring is compressed before it is able to stop the mass. Does D=d. If not, why not. 2. Relevant equations 3. The attempt at a solution This problem really confused me. I got the right answer, but I don't know how to explain the difference. Basically, I started by doing: k*d=m*g, thus k=mg/d Ug1=Ug2+Uspring Ug1-Ug2=Uspring Mg*D-mg*0=(1/2)(mg/d)D^2 1=D/2d D=2d This is what I calculated should happen for the dropping, but I don't understand why it goes further conceptually and hence can't answer why D does not equal d. Any tips/explanations? I couldn't find anything in the chapter of the book that dealt with anything like this.