Elastic potential energy 的 formula 是 1/2kx2 or 1/2Fx, where k = spring constant (Nm-1) and x = distance (m)
based on the first formula, 1/2kx2 , units = Nm-1 * m * m = Nm a.k.a. work done so it becomes joule
the second formula 也是一样
做法就是：E = 1/2kx2
0.56 = 1/2k(0.04)2
k = 0.56*2/(0.04)2
k = 700Nm-1
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