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**1. Homework Statement**

A spring has a relaxed length of 7 cm and a stiffness of 200 N/m. How much work must you do to change its length from 10 cm to 14 cm?

**2. Homework Equations**

Ef = Ei + W

**3. The Attempt at a Solution**

Ef = Ei + W

Ef = Ei + F*delta x

F*delta x = Ef - Ei

F*delta x = (Ki + Ui) - (Kf + Uf)

F*delta x = (1/2mvi^2 + 1/2ks si^2) - (1/2mvf^2 + 1/2ks sf^2)

I'm trying to break this equation apart to try to figure this out, but I'm not sure what to do next. Help!

Thanks