고체역학 (Gere) — Torsion: Problem R-3.3

R-3.3 A brass bar twisted by torques \( T \) acting at the ends has the following properties: \( L = 2.1 \text{ m} \), \( d = 38 \text{ mm} \), and \( G = 41 \text{ GPa} \). The torsional stiffness of the bar is approximately: (A) \( 1200 \text{ N}\cdot\text{m} \) (B) \( 2600 \text{ N}\cdot\text{m} \) (C) \( 4000 \text{ N}\cdot\text{m} \) (D) \( 4800 \text{ N}\cdot\text{m} \)