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

Ui = Uf – Ws
Ws = 0.5kxf2 – 0.5kxi2

3. The attempt at a solution
Assuming Ui is when the spring isn’t stretched.
Uf = Ws
(.55kg)(9.8m/s/s)(-0.2m) = Ws
(.55kg)(9.8m/s/s)(-0.2m) = .5(270N/m)(0.2m)2
Except they aren’t equal.
When Ws = 0.5kxf2 – 0.5kxi2 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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