High School Does d'Alembert Principle Hold Without External Forces in a Galactic Frame?

[MPavsic]

TL;DR

Second and Third Newton Law

Hi, I am brainstorming about Dark Energy.

Lagrange–d'Alembert principle is stating that sum of external forces on the system of rigid bodies and sum of system forces must be in quasi static equilibrium.

What if there are no external forces acting on system of bodies and vector summation, relative to fixed reference frame of all m*a in the system of bodies is not equal to zero; would d'Alembert principle still hold?

 

[MPavsic]

I have to add; Fixed reference frame is attached to the center of the Galaxy.