Editing Torque (section)

=== Moment arm formula ===
[[File:moment arm.svg|thumb|right|Moment arm diagram]]
A very useful special case, often given as the definition of torque in fields other than physics, is as follows:

<math display="block">\tau = (\text{moment arm}) (\text{force}).</math>

The construction of the "moment arm" is shown in the figure to the right, along with the vectors '''r''' and '''F''' mentioned above. The problem with this definition is that it does not give the direction of the torque but only the magnitude, and hence it is difficult to use in three-dimensional cases. If the force is perpendicular to the displacement vector '''r''', the moment arm will be equal to the distance to the centre, and torque will be a maximum for the given force. The equation for the magnitude of a torque, arising from a perpendicular force:

<math display="block">\tau = (\text{distance to centre}) (\text{force}).</math>

For example, if a person places a force of 10&nbsp;N at the terminal end of a wrench that is 0.5&nbsp;m long (or a force of 10&nbsp;N acting 0.5&nbsp;m from the twist point of a wrench of any length), the torque will be 5&nbsp;N&sdot;m – assuming that the person moves the wrench by applying force in the plane of movement and perpendicular to the wrench.

[[File:PrecessionOfATop.svg|thumb|right|The torque caused by the two opposing forces '''F'''<sub>g</sub> and −'''F'''<sub>g</sub> causes a change in the angular momentum '''L''' in the direction of that torque. This causes the top to [[precess]].]]