Elastic potential energy formula requires spring constant and natural length

Steps:

• Identify the formula for spring potential energy: $U = \frac{1}{2} k x^2$, where $x$ is the displacement from natural length.
• Note the given length is 70 mm, but natural length $L_0$ is unknown.
• Without $L_0$, displacement $x = 70 - L_0$ mm cannot be calculated.
• Spring constant $k$ is also missing, preventing numerical solution.

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