When a force acts on a body, it can potentially produce two effects: translation of the body in the direction of the force, and rotation of the body about an axis. For a body in equilibrium, we say that forces have a tendency to produce translation or rotation since no actual acceleration or motion occurs.
The rotational tendency of a force is the subject of this chapter. Engineers refer to this rotational tendency as a moment, short for “moment of a force.” You may remember using the term torque for the same quantity in physics. Physicists use torque specifically for the moment of a force, and moment for the product of any physical quantity with distance. To an engineer, a torque refers specifically to a moment about the long axis of an object that produces twisting and torsional stresses.
A wrench provides a familiar example. A force \(\vec{F}\) applied to the handle of a wrench, as shown in Figure 4.0.1, creates a moment \(\vec{M}_A\) about an axis perpendicular to the page through the center of the nut at point \(A\text{.}\) The \(\vec{M}\) is bold because moments are vector quantities, and the subscript \(A\) indicates the axis or center of rotation. The direction of the moment can be either clockwise or counterclockwise, depending on how the force is applied. If the nut is frozen, no actual rotation occurs, but the force still produces a tendency to rotate it, i.e. a moment.
This interactive shows how a force \(\vec{F}\) causes a moment \(\vec{M}_A\) around point \(A\text{.}\) Rotate the wrench and force \(\vec{F}\) to see how the magnitude of the moment changes.
