Torsion basics – definition:\nThe product of the tangential (circumferential) force acting on a shaft and its radial distance from the shaft axis is called what?

Introduction / Context:
Rotating machine elements such as shafts transmit power through torsion. The fundamental measure that relates the force causing twist to its lever arm is the twisting moment, commonly called torque. Correct terminology distinguishes torque from bending moment, which acts in a different plane and produces curvature rather than twist.

Given Data / Assumptions:

• A circular shaft carries a tangential force at some radius r from its center.
• The force acts perpendicular to the radius, creating a couple about the shaft axis.
• Static equilibrium and small-deformation torsion theory apply.

Concept / Approach:
By definition, a moment (or couple) equals force multiplied by the perpendicular distance to the axis. When the force is tangential around a shaft and the distance is the shaft radius, the resulting moment twists the shaft. This is the twisting moment or torque, denoted T.

Step-by-Step Solution:

Verification / Alternative check:
Bending moment would involve a transverse force producing curvature about a neutral axis, not a circumferential force about the shaft axis. Flexural and torsional rigidities are stiffness measures (E * I and G * J respectively), not moments.

Why Other Options Are Wrong:

• Bending moment: corresponds to bending, not torsion.
• Torsional rigidity: equals G * J, a stiffness parameter with units of force * length^2, not a moment from F * r directly.
• Flexural rigidity: equals E * I, relevant to bending stiffness.

Common Pitfalls:
Mixing torque with power. Remember, power P = T * omega, where omega is angular speed; torque alone is independent of speed.

Final Answer:
Twisting moment (torque)