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torque, in physics, that which tends to change the rate of rotation of a body; also called the moment of force. The torque produced by rotating parts of an electric motor or internal-combustion engine is often used as a measure of its ability to do useful work. The magnitude of the torque acting on a body is equal to the product of the force acting on the body and the distance from its point of application to the axis around which the body is free to rotate. Only the component of the force lying in the plane of rotation and perpendicular to the radius from the axis of rotation to the point of application contributes to the torque. This radius is called the moment arm, or lever arm. The net torque acting on a body is always equal to the product of the body's moment of inertia about its axis of rotation and its observed angular acceleration. If a body undergoes no angular acceleration, there is no net torque acting on it. Units of torque are units of force multiplied by units of distance, e.g., newton-meters, dyne-centimeters, and foot-pounds (or pound-feet).
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