 # Engineering Statics: Open and Interactive

## Section4.1Direction of a Moment

In a two-dimensional problem the direction of a moment can be determined easily by inspection as either clockwise or counterclockwise. A counter-clockwise rotation corresponds with a moment vector pointing out of the page and is considered positive.
In three dimensions, a moment vector may point in any direction in space and is more difficult to visualize. The direction is established by the right-hand rule. Recall that in Subsection 2.8.1 that you were introduced to the right-hand rule and cross products.
To find a moment using the right-hand rule, first establish a position vector $$\vec{r}$$ pointing from the point of interest (the rotation center) to a point along the force’s line-of-action. Next, there are two options for physically finding the direction of the moment from the right-hand rule, the three-finger or slide-and-curl methods.
To use the three-finger method, align your right-hand index finger with the position vector and your middle finger with the force vector, then your thumb will point in the direction of the moment vector. Alternately, if you align your thumb with the position vector and your index finger with the force vector, then your middle finger points in the direction of the moment vector $$\vec{M}$$
Another approach is the point-and-curl method. Start with your right hand flat and fingertips pointing along the position vector $$\vec{r}$$ pointing from the center of rotation to a point on the force’s line of action. Rotate your hand until the force $$\vec{F}$$ is perpendicular to your fingers and imagine that it pushes your fingers into a curl around your thumb. In this position, your thumb defines the axis of rotation, and points in the direction of the moment $$\vec{M}\text{.}$$
Consider the page shown below on a horizontal surface. Using these techniques, we see that a counter-clockwise moment vector points up, or out of the page, while the clockwise moment points down or into the page. In other words, the counter-clockwise moment acts in the positive $$z$$ direction and the clockwise moment acts in the $$-z$$ direction.