Consider the two forces acting on the box in the diagram on the right. The net force is zero because the forces are equal and in opposite directions. The box rotates, however, because there is a torque created by these forces. A torque is a twisting action that may be created by forces depending on where they act. Read the text aloud Torque is created by forces that act to twist or rotate an object
A force creates torque about a particular center point when its line of action does not pass through the center point. The line of action is an imaginary line in the direction of a force passing though the force’s point of application on an object. Both forces in the illustration at right create torque. Read the text aloud
Torque is usually represented by the lower case Greek letter τ (“tau”). The torque τ is the length of the lever arm r times the magnitude of the force F. Read the text aloud
(8.2) τ=r×F
τ  = torque (N m)
r  = distance from center to line of action (m)
F  = force (N)
The lever arm is the perpendicular distance from the center of rotation to the line of action of the force. The torque created by a particular force depends on the length of the lever arm. A longer lever arm means larger torque even through the force may not change. When the line of action passes through the center of rotation, however, the lever arm is zero. In this case the torque is also zero no matter how large a force is applied. Read the text aloud
The units of torque are force times distance, or newton-meters. A torque of 20 N m is created by a force of 100 N acting with a lever arm of 0.2 m. Because torque is a product of two variables, it is possible to create the same torque with different forces. For example, a 20 N force applied with a lever arm of 1 m also produces a torque of 20 N m. Read the text aloud The line of action lies in the direction of the force. Show Adding torques together
In English units, torque is usually expressed in pound-feet. A torque of one pound-foot represents the twisting action of a one pound force with a lever arm of one foot. Read the text aloud Show Foot-pound or pound-foot?
Jamir is struggling to loosen a bolt with a short wrench. If he uses a longer wrench but pushes with the same force, what happens to the torque generated?
  1. It increases.
  2. It decreases.
  3. It does not change.
  4. No torque is generated.

238Previous Page Next Page