An object is at equilibrium at a certain time t if the sum of the forces and torques = 0. Look at the object from a moving frame, and apply the equations of special relativity. We find that Lorentz contraction means that in the moving frame (x',t'), there is no time t' such that the object is at equilibrium. Is the Lorentz contraction valid, or what? Doesn't equilibrium mean that in the moving system we look at the sum of the forces and torques at a certain time t'?