Torque, moment or moment of force, is the tendency of a force to rotate an object about an axis,fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.
Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.
Torque and moment are often used interchangeably. Most people are confused when asked the difference between moment and torque. The terms torque and moment originated with the study conducted by Archimedes on levers. Torque (most commonly used) or moment (used by engineers) is a concept of turning force. This turning force is applied when we push a door or try to open a nut using spanner. Both the door and the spanner turn about a point called the pivot or fulcrum. The force that is applied is at some distance from this fulcrum. The turning effect of the force applied depends upon this distance from the pivot or fulcrum.
Moment = Force* Perpendicular distance from the pivot
From this equation, it is clear that if we want to complete the task using less force, we have to increase the distance from the pivot.
In contrast, when a car driver turns a steering wheel, he exerts two equal and opposite forces on the steering. These forces form a couple and the turning effect of this couple is the sum of the moment of the two forces.
The moment of a couple is called Torque.
Torque= Force*Perpendicular distance between the two perpendicular forces
In common parlance, Torque and moment are used interchangeably. Torque, or the moment of a force is its ability to rotate an object about an axis. While force is applied in torque as well, force is a push or a pull but in torque this force is in the form of a twist.
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For the students of mechanical engineering, the two terms are different and not interchangeable. In general moment is the term used when referring to the ability of a force to turn an object about its axis. Torque is a special application of moment. When there are two equal and opposite forces, they form a couple, and the moment that results is called a torque. Here the applied force vectors are zero. 