Engineering Statics: Open and Interactive

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 call this rotational tendency a moment, short for “moment of a force.” You may remember the using the term torque for the same quantity in physics. Physicists use torque specifically for a moment of a force, and moment for the product of any physical quantity with distance. To an engineer, a torque means a moment about the long axis of an object that produces twisting and torsional stresses.

A wrench provides a familiar example. A force $\mathbf{F}$ applied to the handle of a wrench, as shown in Figure 4.0.1, creates a moment ${\mathbf{M}}_A$ about an axis perpendicular to the page through the center of the nut at $A\text{.}$ The $\mathbf{M}$ 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 counter-clockwise 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.