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For a hollow circular shaft (outer dia D, inner dia d), polar moment of inertia J =

ARandom
Bπ(D⁴ - d⁴)/32
CπD⁴/32
Dπd⁴/32
Answer & Solution
Correct answer: B. π(D⁴ - d⁴)/32
**Hollow shaft:** J = π(D⁴ - d⁴)/32. Solid shaft: J = πD⁴/32. Polar section modulus Z_p = J/(D/2). Hollow shafts more efficient than solid (most torque carried by outer fibres). Specific torque (T/weight) higher for hollow.
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