# Potential energy and a spring

**1. The problem statement, all variables and given/known data**

How much work is done by the spring on the object as it stretches 0.02m? The spring has a k value of 270N/m and the object is 0.55 kg.

**2. Relevant equations**

U_{i} = U_{f} – W_{s}

W_{s} = 0.5kx_{f}^{2} – 0.5kx_{i}^{2}

**3. The attempt at a solution**

Assuming U_{i} is when the spring isn’t stretched.

U_{f} = W_{s}

(.55kg)(9.8m/s/s)(-0.2m) = W_{s}

(.55kg)(9.8m/s/s)(-0.2m) = .5(270N/m)(0.2m)^{2}

Except they aren’t equal.

When W_{s} = 0.5kx_{f}^{2} – 0.5kx_{i}^{2} is used, the answer is -0.054J. What is wrong with my first method?

**1. The problem statement, all variables and given/known data**

**2. Relevant equations**

**3. The attempt at a solution**

http://ift.tt/1pORtfg

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