Torsional rigidity identification:\nFor a prismatic circular shaft, which expression represents torsional rigidity (torque per radian of twist over the full length)?

Introduction / Context:
Torsional rigidity characterizes how resistant a shaft is to twisting under applied torque. It is central to shaft design for power transmission, where angular deflection limits are specified in addition to strength.

Given Data / Assumptions:

• Straight, prismatic shaft, length L, polar second moment J, shear modulus G.
• Linear elastic torsion: θ = T * L / (G * J).

Concept / Approach:
Rearranging the torsion formula shows torque per radian of twist (stiffness) equals GJ/L. Since T/θ = GJ/L, the expression T/θ directly represents torsional rigidity over the entire length (units: N·m/rad).

Step-by-Step Solution:

Verification / Alternative check:
Dimension check: GJ/L has dimensions of torque per radian, consistent with stiffness. Numerical examples confirm proportionality between T and θ for given G, J, and L.

Why Other Options Are Wrong:

• T/J and T/r are not stiffness measures.
• T/G has incorrect dimensions.
• G * J / L is a correct explicit form, but the option asked for an expression in terms of T and θ; thus T/θ is the matching choice.

Common Pitfalls:
Confusing section property J with polar radius r; mixing strength (τ = T r / J) with stiffness (θ relation).

Final Answer:
T / θ