Angle of Twist Illustrated  
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Torque T is applied on a shaft of length L at either ends in opposite directions. The radial and longitudinal lines are shown on the shaft. The deformation of the shaft is shown with a counterclockwise segment of angle phi, angle of twist.

When the torque T is applied, the right-hand end of the shaft will rotate with respect to the left-hand end. This angle of rotation is known as the angle of twist ϕ.

The angle of twist changes along the length L of the shaft. For a prismatic shaft, the angle of twist will vary linearly between the left-hand and right-hand ends.