Right now I'm reviewing old AP problems for the AP exam for Physics B tomorrow, and I can't seem to get a simple spring problem. I don't know why, but I can't just grasp it. Here it is: A block of mass 3kg is hung from a spring, causing it to stretch 12 cm at equilibrium, as shown above. The 3 kg block is then replaced by a 4 kg block, and the new block is released from the position shown above, at which the spring is unstretched. How far will the 4 kg block fall before its direction is reversed? Here's my work(I'm allowed to use 10m/s^2 for gravity): 3kg(10m/s^2) + .12k = 0 .12k = 30 k = 250 250x - 4(10) = 0 x = .16 m = 16 cm The answer I'm told is supposed to be 32... can someone explain what I'm doing wrong?