如右圖所示，軸向受力之圓桿，求 P 力作用下，總應變能最接近下列何者？（已知 E = 200 GPa、d = 0.2 m、L = 2 m、P = 50 kN）

U = P²L/(2AE)。A = π×(0.2)²/4 = 0.031416 m²；分子 = (50000 N)²×2 m = 5×10⁹ N²·m；分母 = 2×0.031416×200×10⁹ = 1.2566×10¹⁰ N·m；U = 3.979×10⁻¹ N·m = 0.3979 J = 0.3979 N·m × (1kN/1000N) × (1000mm/1m) = 0.3979 kN·mm ≈ 0.398 kN-mm，答案正確。