Question about potential for this mechanical force problem

[Zouatine]

Hi! hope you are doing well ,

I'm trying to solve this problem ( mechanical problem) about structure with load F as it showing in the next figure , i want to determine to critical force F , the stability force ( which means if my load is bigger than the critical load my system will be unstable) ,so to get i have to do by Energy method which means i have to use potential ,in this step i have some questions :

1- why the External Energy must be with negative sign ?
2 - what does mean the total potential is Stationary ?

 

[anorlunda]

This sounds like homework. It it?

 

[Zouatine]

hi thank you for your comment!

,no it's not , solution of the exercise i already know it , i said i have to use Energy method , which mean the first step to solve it : Total Energy = Internal Energy + External Energy ,
Internal Energy is the energy in the Spring , and for External Energy is the Energy cause by external Force , which it's = Force * Displacement ( cause by this force F), but my question here why we have to do sign mince in the formula of External Energy because the formula must be External Energy = - Force * Displacement, ?
when we get the total Energy we must to derivation of Total Energy corresponding to displacement =0 .
d(Total Energy )/d(displacement) =0 because the potential is Stationary , and here is my second question what does mean the potential is Stationary ?

i know the process how to solve but i have these two questions , thank you hope my problem is clear now

 

[Dr.D]

What exactly do you mean by "External Energy"? This is not a term I've ever heard before, so perhaps you could define it before we try to talk about it.

 

[Zouatine]

hello , thank you for your comment , sorry i made a mistake , i want to say external work , External Work= Work has done by the External Forces , Thanks and sorry again ,