Moment of a Force about a Point and an Axis:

The moment of a force may be determined with respect to (about) a point and also about line of axis.

The moment of a force F about a point A is described as a vector with a magnitude equal to the product of the perpendicular distance from A to F and the magnitude of the force and along a direction perpendicular to the plane having A and F. The sense of the moment vector is given by the direction a right-handed screw would advance along with if turned around A in the direction mention by F as illustrated in Figure.

The moment of a force with respect to a line or axis perpendicular to a plane containing the force is described as a vector along with a magnitude equal to the product of the magnitude of the force and the perpendicular distance from the line to the force and along with a direction along the line. Thus, it is the same as the moment of the force w.r.t. the point of intersection of the plane & the moment axis.

As the moment of a force w.r.t an axis is a measure of its tendency to turn or rotate a body around the axis, the force parallel to an axis has no moment about the axis, because it has no tendency to rotate the body w.r.t. the axis.

The moment of a force around various points and axes is shown in Figure. The moment of the horizontal force F w.r.t. point A has a magnitude of Fd₁ in the direction

Shown by M_A (Figure (a)). Likewise, M_B, the moment w.r.t. point B, contains a magnitude of F_d2 and is perpendicular to the plane find out by B and the force F. The moment of force F w.r.t. the line AB is the same as M_A (as illustrated in Figure (a)) or M_AB (as illustrated in Figure (b)). The moment of force F around the line BC may be obtained by resolving force F into components F₁ and F₂. As F₁ is parallel to line BC, it has no moment around BC. The resultant F₂ is in a plane perpendicular to BC and its moment is F₂d₃ in the direction illustrated. Likewise, the moment around line BD is F₁d₃ as mentions. Here, you might note down that M_AB, M_BC and M_BD are the orthogonal components of MB.