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Torque

The turning effect of a force about the axis of rotation is called moment of force or torque due to the force.
 
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Fig. 4
 
If a particle is rotating in xy plane about the origin under the effect of force 43702.png, and at any instant the position vector of the particle is 43695.png (Fig. 4), then
 
Torque, 43689.png
 
or τ = rF sin φ
 
where φ is the angle between the direction of 43683.pngand 43677.png.
  • Torque is an axial vector, i.e., its direction is always perpendicular to the plane containing vector 43670.png and 43664.png in accordance with right hand screw rule. In Fig. 4, the sense of rotation is anti-clockwise so the direction of torque is perpendicular to the plane, outward through the axis of rotation.
  • Torque is the cause of rotatory motion and in rotational motion it plays the same role as force plays in translatory motion, i.e., torque is the rotational analog of force. This all is evident from the correspondences between rotatory and translatory motion given in Table 2.
Table 2
 
Rotatory motion
Translatory motion
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