Potential Energy: Differentiating to Find Force

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omarMihilmy
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To find the force we will differentiate the function with respect to x (dx)
My problem is with the solution
If we differentiate:
dU/dx = 9x^2 - 7

From where does the negative come from also the j component?
 
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You should have used the homework template, then you have written the relevant equations. What equation is relevant for finding force from potential energy?
 
That is not correct.
 
Correct me if I am wrong
 
Why are you disregarding the y-component when you differentiate?