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Bending stress–section modulus relation in beams For a prismatic beam subject to bending moment M, use the flexure formula. Choose how bending stress varies with section modulus.

Difficulty: Easy

Directly proportional to section modulus
Inversely proportional to section modulus
Equal to section modulus
Independent of section modulus

Correct Answer: Inversely proportional to section modulus

Explanation:

Concept/Formula
Flexure formula: σ = M y / I. Section modulus Z = I / y. Therefore, σ = M / Z.

Implication
For a given bending moment M, increasing Z (by choosing a stiffer cross-section) reduces the bending stress linearly.

Final Answer
Inversely proportional to section modulus.

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