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Mi question is this: in rational mechanics, how do i decide the sign of a potential?

Let me explain better: in some exercises it may happen that a mass particle is subject to both gravity and, for example, the force of a spring. If by hypothesis, the two forces are always perpendicular, how should i write the potential?

V= mgy -1/2*k*x^2 or V=mgy + 1/2*k*x^2

Let me underline that this question is not trivial!!! Infact that mere sign can change everything, including the stability of equilibria points!!

I'd be really grateful to anyone who can answer this question.

If i wasn't clear in explaining my doubts, please fell free to tell me.