Definition of Torque
Force F is applied to some point on a rigid object. The line of action is a line thru the application point parallel to the force.
The Torque due to the force is defined by:
where is the “lever arm” or “moment arm” is the perpendicular distance from axis to line of application.
Notes:
We can think about torque as a rotational analogue of force. A force can more effective in producing rotation both because it is large and because it is has a large moment arm. That is, if you want to make something rotate around an axis, it helps to apply the force in such a way that the line of action of the force passes far away from the axis. When you push open a door, for example, it’s better to push perpendicular to the door as far away from the hinge as you can manage. For the same reason, it’s a lot harder to turn a very small doorknob than it is to turn a big one. In both cases, you don’t need a big force if you have a big moment arm.