1. The problem statement, all variables and given/known data Everything is on a frictionless horizontal surface. If a block is compressed against a spring for some distance x meters and then released, what is the velocity of the block the moment it loses contact with the spring? 2. Relevant equations W_s = -(1/2)kx^2 [if reference point is 0m] W = (1/2)m(v_f)^2 - (1/2)m(v_i)^2 3. The attempt at a solution So if we take the compression of the spring to be our 0 point, then when it is released, the block travels x meters when it is fully released by the spring. The work done by the spring on the block is going to be negative. But since the initial velocity is 0, we get the equation: -(1/2)kx^2 = (1/2)m(v_f)^2. If you try to solve this for the velocity, you have to take the square root of a negative number. What am I doing wrong here?