English 301/901 Lesson 2

Chapter 13: Defining

Torque

Imagine pushing a door to open it. The force of your push causes the door to rotate about its hinges. How hard you need to push depends on the distance you are from the hinges (and several other things that will be regarded as negligible). The closer you are to the hinges (i.e. the shorter the distance you are from the hinges), the harder it is to push. This is what happens when you try to push open a door at the wrong end, too close to the hinges. The torque you create on the door is smaller than it would have been had you pushed the correct side, away from it’s hinges.

Fig. 1 Fig. 2 Pythagorean Theorem

Torque is the ability of a force to cause a body to rotate around a particular point, or axis. It is a measure of the “twist” produced by a force about the particular axis (also known as the moment about the axis). It is a vector, meaning that it has a direction as well as a magnitude (i.e. 50km/h south rather than just 50km/h).

Torque can have counterclockwise or clockwise rotation, defined to be negative and positive, respectively. Torque is equal to r x Fsinq, where r is the distance drawn from the pivot (i.e. the hinges of the door) to the F, the applied force (i.e. where you’re pushing on the door to open it). Only the force being applied perpendicular (at a right angle) to the object affect it, therefore, using Pythagoras’ theorem (see Fig. 2) we multiply the force by sin q to convert to the perpendicular component of the force. q is the angle between the force and the object. The units for torque are Newton-meters (Nm). The symbol for torque is the Greek letter tau, t , pronounced to rhyme with how. If an object is acted on by more than one force, the total torque is the sum of the torques due to the individual forces. Torque increases with the distance from the pivot to the force application point. Greatest torque is also achieved when the force is at a right angle to the object.

Imagine using a wrench to loosen a bolt. The force is applied where you hold the wrench and the bolt is the pivot (see Fig.3).

If an object is in rotational equilibrium, the sum of the torques is equal to zero and there is no net torque on the object. Torque is important for things like tightening the wheel nuts on your car, maybe after changing a flat tire. There are specifications of the tightening torque to ensure the nuts won’t come loose. Mechanics often use a “torque wrench” that provides the direct indication of the applied torque.