Stress is defined as what, in terms of internal forces?

• A. Force per unit area
• B. Torque
• C. Temperature change
• D. Mass density

Answer: (A)

Stress is the internal force that resists deformation within a material, defined as force per unit area across a cross-section. In a bridge member under load, the material develops internal forces to carry the applied loads, and stress quantifies how those forces are distributed over the cross-sectional area. That’s why “force per unit area” is the best description. Torque describes a rotational effect, not the basic measure of internal force per area. Temperature change leads to expansion or contraction and related strain, but by itself is not stress. Mass density is simply mass per volume and doesn’t describe internal forces. Stress is expressed in units like N/m^2 (Pascals) or psi.