What is torque?

Torque is an essential concept in mechanics, describing the rotational equivalent of linear force. It is the measure of the force that can cause an object to rotate about an axis, with the direction of the torque vector depending on the direction of the force and the angle between the force vector and the moment arm.

Torque can be either static or dynamic. A static torque refers to a force applied on an object that does not produce angular acceleration, such as pushing on a closed door or pedaling a bicycle at constant speed.

On the other hand, a dynamic torque refers to a force that is producing angular acceleration, such as the drive shaft in a racing car accelerating from the start line.
The magnitude of torque is calculated by multiplying the force by the length of the moment arm and the sine of the angle between the force vector and the moment arm. The direction of the torque vector can be determined by using the right-hand grip rule, where a person curls their hand around the axis of rotation with their fingers pointing in the direction of the force and the torque vector points in the direction of their thumb.

The unit of torque in the SI system is the Newton-meter, while in the imperial system, the Foot-pound is commonly used. Measuring torque in a non-rotating system is relatively straightforward and can be done by measuring force and determining the length of the moment arm. However, measuring torque in a rotating system is more challenging and usually involves measuring the strain within the metal of a drive shaft and transmitting this information wirelessly.
In rotational kinematics, torque takes the place of force in linear kinematics, with the equation of motion being τ = Iα, where α is the angular acceleration and I is the rotational inertia. The rotational inertia depends on the mass distribution of the object and determines how difficult it is for an object to acquire angular acceleration. Understanding torque and its role in rotational kinematics is crucial in various fields, including engineering, physics, and mechanics.