Fundamentals

The amount of torque caused by a force is dependent upon the lever arm (a.k.a., moment arm). The lever arm is the shortest distance between the axis of rotation and the line of force.

 

There are four similar versions of this question. Here is one of the versions:

Version 1:
Anita Getin pulls on the handle of a door to swing it about its hinge (⦻). The direction of the force vector is shown in red (and labeled F). The torque (τ) resulting from this force is calculated using the equation   τ = a*F   where a is the lever arm (a.k.a. moment arm). Which of the blue arrows - labeled A, B, C, D, and E - represent the lever arm?
 
 
 
 


 

How to Think About This Situation

When a force is applied to an object in a direction that does not extend through the axis of rotation, a torque is said to exist. In this diagram, a force is applied to a door knob to rotate the door about the door hinge. The amount of torque resulting from this force is dependent upon the length of the lever arm. The lever arm is the shortest distance between the axis of rotation and the line of force. So consider a straight line extending in both directions through the force arrow; this is the line of force and is shown as a dashed extension line on the diagram. Then consider a line drawn from the axis of rotation (door hinge) perpendicular to the line of force. Such a perpendicular line represents the shortest distance to the line of force. The length of this line is known as the lever arm. 

 

Try these links to The Physics Classroom Tutorial for more help with understanding the concept of torque and rotation:

​Sorry. We hope to add resources in the future.
 


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