MOMENTS AND TORQUE

When more than one force acts on different parts of a body, there is the possibilty that, ,instead of or as well as, producing motion in the direction of the resultant force, the body will be rotated. The torque or moment of a force is a measure of the turning affect of a force at a point. It is equal to the magnitude of the force multiplied by the perpendicular distance of the point from the line of action of the force. T = F * r Hence the dimensions of torque are [M]¹ [L]² [T]^-2. Values of moment are given a +/- value depending on whether they tend to produce clockwise or anticlockwise rotation.

COUPLES
As has already be said, to produce rotation there need to be at least two forces, if these two forces only tend to produce turing motion, then their are refered to as a force couple. For example when you open a door, the force you apply to the handle and the reaction of the hinges are a force pair then tend to produce rotation in the door. Because couples must only produce rotation and no translational motion, there magnitudes are always equal and the direction opposite.

Notes:

• The torque due to a force couple is equal to their magnitue multiplied by the perpendicular distance between their lines of action.
• If the vector sum of all forces acting on a body is zero it will not have any translational motion.
• If the sum of moments about any point in a body is zero it will not rotate.