The Right-Hand Thumb Rule
When dealing with torques in a 2-dimensional \(x\),\(y\) or \(x\),\(z\) plane, we know that rotations counterclockwise are considered positive, while rotations clockwise are considered negative.
This simple and memorable mnemonic, however, reaches its limitations when the problem becomes 3-dimensional and equilibrium conditions need to be established around all three axes.
However, there is another even simpler and more memorable rule to quickly and reliably determine the positive rotational direction of a moment acting on any arbitrary axis:
Right-hand thumb rule
If you point your right thumb in the positive direction of the axis around which the rotational direction of a moment is sought, the curved fingers of your right hand indicate the positive rotational direction.
One can easily remember the grip required for this rule by picturing the right thumb as a screwdriver being held by the other fingers of the right hand, tightening a screw.
Figure 1 demonstrates the validity of the case described above for a 2D \(x\), \(y\) coordinate system: The counterclockwise direction of a moment is considered positive.
Naturally, this rule applies to all axes.
The rule can also be used to determine the sense of rotation of a moment represented by a double arrow:
The right-hand thumb rule for double arrow moments
If the extended thumb points in the direction of the double arrow, then the remaining curved fingers indicate the direction of the moment.
