Which term refers to the internal reaction of a material under external load?

• A. Stress
• B. Strain
• C. Elasticity
• D. Plasticity

Answer: (A)

When external forces act on a material, internal forces develop inside the material to resist and balance those forces. The quantity that describes this internal reaction per unit area is called stress. It’s like the material’s internal “force per area” response to the load, often written as sigma, with units of pascals (N/m^2). For axial loading, stress = force / cross-sectional area, capturing how intensely the material is resisting the applied load.

Strain is different: it measures how much the material actually deforms, as a change in length relative to its original length. Elasticity and plasticity describe how the material behaves after loading—elasticity is the tendency to return to the original shape when the load is removed, while plasticity is permanent deformation after yielding. Since the question is about the internal reaction to an external load, the term is stress.