One-half length of 50 mm diameter steel road is solid while the remaining half is hollow having a bore of 5 mm. The rod is subjected to equal and opposite torque at its ends. If the maximum shear stress in solid portion is $\tau$, the maximum shear stress in the hollow portion is :
- A. 15/16 $\tau$
- B. $\tau$
- C. 4/3 $\tau$
- D. 16/15 $\tau$
Answer: B. $\tau$
Explanation: The outer diameters are same (50mm). Since the bore (5mm) is very small ($d = 0.1D$), the polar moment of inertia $J$ is almost identical ($1 - 0.1^4 \approx 1$). Thus stress is approximately equal.