I have a question about thse following system: block of mass M is attached to a spring is on an inclined plane. Why do the force and energy analysis equations give different answers? The question is to find the maximum extension of the spring. Here is my thinking so far. I know if you use energy convervation, you get 1/2*k*x^2 = M*g*sin(theta), so you get x = 2*M*g*sin(theta)/k. However, why can I not just set the force along the incline equal to the spring force and solve for x that way? If I do that, I get a different answer: k*x = M*g*sin(theta) x = M*g*sin(theta)/k and this is different by a factor of 2.