I would like to know how to properly write the elastic force in the classic x and y axes. I know that the global formula is $\vec{F_{el}}= -k\vec{x}$, but sometimes the situation is little bit difficult. Here is an example of a situation where I struggle :

Considering here two elastic forces (springs of k coefficient and $L/2$ undeformed length) with the origin $O$ in the middle (A and B slide), for the "left" elastic force I write it this way : $\vec{F_{el_A}}= -klsin(\phi)\vec{i}$ for the "right" one my guess is $\vec{F_{el_B}}= -klsin(\phi)\vec{i}$ too. I do not know if I am right but the elastic energy says that $V_A(\phi)=\frac{1}{2}k(lsin\phi)^2$ but $V_B(\phi)=\frac{1}{2}k(-lsin\phi)^2$ which makes me think I was wrong for the forces.

Plane Oij

In general, how to write properly the elastic force and eventually elastic energy ?

  • $\begingroup$ There might be something with the sign of the force analysis. If the object $m$ is stationary (not moving), then the spring forces must be opposite and then they must have opposite signs. $\endgroup$
    – Steeven
    Jan 6, 2021 at 20:24
  • $\begingroup$ The object m moves freely $\implies \phi \in [0,2\pi]$ and the extremities A and B slide on the walls so the springs are always horizontal $\implies$ the elastic forces are always horizontal too, I would like to know if the elastic forces are equal in this case $\endgroup$
    – Kilkik
    Jan 6, 2021 at 20:27
  • 1
    $\begingroup$ The forces are in the same direction. For the energy, the direction does not matter. $\endgroup$
    – nasu
    Jan 6, 2021 at 21:40


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