Here's why:
* Equilibrium in physics means that an object is at rest or moving at a constant velocity. This occurs when the net force and net torque acting on the object are both zero.
* Forces cause changes in motion (acceleration). When the forces are balanced, the object will not accelerate.
* Moments (or torques) cause changes in rotation. When the moments are balanced, the object will not rotate.
Therefore, when the sum of forces and moments in a structural system equals zero, the structure is in a state of equilibrium. This means the structure is stable and not undergoing any acceleration or rotation.
