Work by ideal spring force (W_spring = ½k(x₁² − x₂²))

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W_spring = ½k(x₁² − x₂²)

What each symbol means

SymbolWhat it stands for
kSpring constant (N/m)
W_springWork done BY the spring on the block (J)
x_1, x_2Initial and final extensions/compressions from natural length (m)

When to use this

Hooke's law F = −kx from equilibrium; ideal massless spring. x is displacement from natural length (sign matters).