Torsion is caused by which effect?

• A. A force resulting from an external moment that rotates a member about its longitudinal axis
• B. A twisting moment about the longitudinal axis
• C. A force perpendicular to the member
• D. A vertical gravitational weight

Answer: (A)

Torsion is the twisting of a member caused by a twisting moment (torque) applied about its longitudinal axis. This arises from an external moment, essentially a couple of forces, that tends to rotate the cross-sections along the length of the member. That’s why the idea of a force resulting from an external moment that rotates the member about its axis describes torsion.

The other ideas describe bending or vertical loading rather than twisting about the axis. A force perpendicular to the member tends to bend or shear it, not twist it. A vertical gravitational weight acts downward and can produce bending or axial effects depending on orientation, but not pure torsion.