I accidentally posted over in general questions before seeing that wasn't the proper place for this type of question. I'm not looking for the answer, as I have that available. I'm just trying to understand part of the process here. Question: At a waterpark, sleds with riders are sent along a slippery, horizontal surface by the release of a large compressed spring. The spring, with force constant k = 40.0 N/cm and negligible mass, rests on the frictionless horizontal surface. One end is in contact with a stationary wall. A sled and rider with total mass 70.0 kg are pushed against the other end, compressing the spring 0.375 m. The sled is then released with zero initial velocity. What is the sled's speed when the spring (a) returns to its uncompressed length and (b) is still compressed 0.200 m? Solution is attached. My problem: In part a, I understand that "Wspr = (1/2 kx22 - 1/2 kx12)", but I don't understand why it's equal to "- (1/2 kx22 - 1/2 kx12)". The solution says: "The force the spring exerts on an object attached to it is F = -kx so the work the spring does is Wspr = - (1/2 kx22 - 1/2 kx12)". How does the Force being -kx directly related to the spring's total work value also being negative?