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## Homework Statement

The stretch of a nonlinear spring by an amount x requires a force F given by:

F=40x-6x^2

where F is in Newtons and x is in meters.

What is the change in potential energy U when the spring is stretched 2m from its equilibrium position?

## Homework Equations

U=.5kx^2

= .5(kx)x

F=kx

## The Attempt at a Solution

F=40(2)-6(2)^2=80-24=56N

U=.5(56N)(2)=56J

Thats what I thought would work, but 56J is no the correct answer. What am I doing wrong?