Torque is simply a twist, and it got its name only in 1884
A push moves things in a line; a twist turns them around an axis. Physicists call that twist torque, engineers call it a moment, and the word itself arrived surprisingly late, in print in April 1884. Its formula shows why the same push twists harder the farther from the pivot it lands.
Torque is the rotational counterpart of force. Drive a screw and the screwdriver applies it, making the screw tend to spin about its axis. Vocabulary depends on place and profession: physicists generally say torque, while mechanical engineers in both Britain and the United States tend to say moment of force, or just moment. That older term goes back at least to 1811, in Simeon Denis Poisson's treatise on mechanics, translated into English in 1842. Symbols follow the same split, with the Greek letter tau for torque and M for moment.
The word comes from the Latin torquere, to twist. James Thomson is said to have proposed it, and Silvanus P. Thompson used it the same year in his book on dynamo-electric machines. Thompson argued that a single notion of twisting a shaft was clearer than talk of couples, moments and forces applied with leverage.
The basic calculation is lever arithmetic. Multiply the part of a force acting at right angles to a lever by the distance from the pivot, and you have the torque, measured in newton-metres. In three dimensions it becomes a pseudovector, the cross product of the position vector and the force. Its size depends on the force, the lever arm and the angle between them, and a force pointing straight along the arm produces no turning at all. The direction comes from the right-hand grip rule: curl your fingers from the arm toward the force and your thumb shows the way the torque points.
Torque plays the same role for spinning that force plays for straight-line motion. The net torque on a body sets how fast its angular momentum changes, a rotational version of Newton's second law, and for a simple rotating disc it equals moment of inertia times angular acceleration. The rate at which torque itself changes even has a name: rotatum.
Source: Torque
