Which statement best defines torsion in mechanics?

• A. The change in length with temperature.
• B. The twisting of an object due to an applied torque.
• C. The bending of a beam under load.
• D. The rotational motion due to friction.

Answer: (B)

Torsion in mechanics is the twisting of an object around its axis when a torque is applied. When you apply a twist, each cross-section along the length rotates a little relative to the one next to it, producing shear deformation and an angle of twist that accumulates along the length. This is exactly what happens in a shaft transmitting power: the motor applies torque, causing the shaft to twist.

This distinguishes torsion from other effects: changing length with temperature is thermal expansion, not twisting; bending a beam under a transverse load creates curvature, not uniform twisting around the axis; and rotational motion due to friction is a different mechanism not characterized by torque-induced shear twisting along the length.