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A measure of the effectiveness of a force or moment in setting a body in rotation. In mechanics, a torque is a twisting moment or couple which tends to twist a fixed object such as a shaft about a rotation axis. If the shaft starts to rotate, the power it transmits is give by the product of the rotational speed and the torque.

An object may be acted upon by multiple torques. The motion of the system then depends on the net torque on the system. If a system is in equilibrium, or balanced, the torques would be equal and no net torque would act on the system.

The term "torque" is often used to describe the motion produced by helicopter rotors. Without a balancing rotor, the torque of a rotor would cause the helicopter to turn in an uncontrolled fashion. Helicopters often use tail rotors to counter the torque produced by the main rotor. Sometimes, helicopters use tandem rotors or coaxial rotors that moving in opposite directions.

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Source: U.S. Centennial of Flight Commission