Final answer:
An external moment in a structure is the external torque applied to cause rotation or bending, and for equilibrium, the net external torque must be zero.
Step-by-step explanation:
An external moment in a structure refers to the torque that is applied externally to a structure. Moment, in this context, is the rotational equivalent of force that can cause rotation or bending of a structure. The magnitude of the torque is given by the formula T = rF sin θ, where T is the torque, r is the distance from the pivot point to the point where the force is applied, F is the magnitude of the force, and θ is the angle between the force and the lever arm direction. For a structure to be in equilibrium, two conditions must be satisfied. The first condition is that the net external force on the system must be zero (net F = 0), and the second condition is that the net external torque must also be zero (net T = 0). This means for any choice of pivot point, in an inertial frame of reference, the second condition is satisfied if the external torques balance out and result in no net torque on the structure.