The moments - also called torque - is a description of the tendency of a body to rotate about an axis when a force is applied. If the object is at equilibrium meaning that it is not rotating, the moment about any point on the object is zero.
The moment quantity is obtained by multiplying the force with the respective lever arm. The lever arm is the distance between the perpendicular applied force on the body and its axis. If the distance is zero, obviously, the moment is also zero. What if the force is not perpendicular? In this situation, the angle between the force and the member needs to be taken under consideration. The moment is obtained through the following mathematical operation:
M = rFsinθ
where r is the distance between the applied force F and the axis, and θ is the angle formed between the member and the applied force. Note that when θ = 90º, sin90 º = 1 and thus, keeping the equation valid. If the force is parallel with the member, θ will be zero with sin0 º = 0 and therefore, the moment will be zero.
The unit of moment is in general expressed in Newton-meter (Nm). It can also be expressed as lb-ft, lb-in, etc.
In case when more than one force is being applied to a member, the overall moment is the sum of all individual moments caused by each individual force. In equilibrium conditions, the sum of all moments equals to zero: