Torque

According to English physicist and mathematician Sir Isaac Newton (1642–1727), an object at rest will remain at rest, and an object will remain in motion unless acted upon by an outside force. A force, therefore, is what causes any object to move. Any force that causes an object to rotate, turn, or twist is called a torque. In physics, torque is often described as a rotational force. Torque is equal to the amount of force (F) being exerted on the object times the object’s rotation point to the location where the force is being applied on the object (r).

Thus, torque (τ) is described by the equation: τ= r× F, where r is the position vector of the object, F is the force vector acting on the object and “X” is the cross product of the two vectors. The SI (International System of Units) unit for torque is newton meters (N-m). Other units for torque include foot pounds-force (ft-lbf) and inch pounds-force (in-lbf). Tau (τ) is the Greek letter for torque. Torque, which is also called momentum, is considered to have been first used when ancient Greek mathematician Archimedes (287–212 BC) was working with levers.

Seesaws are a good example of torque. Many people have had the experience of someone sitting on one end of the seesaw and another person sitting on the other end; and one person is heavier than the other. If the larger person is sitting closer to the pivot point of the seesaw, the smaller person can lift them with little or no problem. The reason this is possible comes from the difference in torque experienced by each person. Even though the smaller person exerts a smaller force, their distance from the pivot point (lever arm) is longer, hence a large torque. The larger person exerts a larger force; however, because he/she is closer to the pivot point their lever arm is shorter, hence a smaller torque.

Wrenches also work by torque. (Some wrenches are even calibrated to display the amount of torque the user is applying to a nut; they are called torque wrenches.) The nut (or bolt) is the point of rotation because the user wants to tighten or loosen it by having it turn. The force is being exerted by the hand and arm. Since users try to pull or push (exert a force) at right angles on the wrench’s handle, the lever arm is then the length of the wrench’s handle. To increase the torque on the nut, the user must either increase how hard the lever arm is pulled or pushed; or, increase the length of the lever arm by placing, for example, a pipe over the end of the wrench.