Stress Defined in Mechanics

Steps:

• Identify stress as a measure of internal force in materials under load.
• Recall stress quantifies force distribution over the material's cross-section.
• Distinguish from strain, which measures deformation relative to original dimensions.
• Match definition to standard formula σ = F/A.

Why C is correct:

• Stress is defined by the formula σ = F/A, where F is applied force and A is cross-sectional area, per Hooke's law in mechanics.

Why the others are wrong:

• A: Extension per unit area describes deformation density, not force; confuses with strain variants.
• B: Extension per unit original length defines strain (ε = ΔL/L), not stress.
• D: "Extension area" is undefined; stress uses original cross-sectional area, not extension-based.

Final answer: C