Basic definition check: the resistance per unit area that develops inside a material when external forces cause deformation is called what?

Introduction / Context:
Understanding the distinction between stress and strain is foundational in mechanics of materials. When loads are applied, a body deforms and internal forces develop to resist that deformation. We quantify this internal resistance using stress.

Given Data / Assumptions:

• A continuum body under external forces.
• Uniform cross-section considered for simplicity.
• Elastic range assumptions for clarity.

Concept / Approach:
Stress is the internal force per unit area that develops on an imagined cut through a body. Normal stress acts perpendicular to the area; shear stress acts tangentially. By contrast, strain is the geometric measure of deformation (change in length per original length, change in angle, etc.).

Step-by-Step Solution:

Verification / Alternative check:
In shear, tau = V / A_s, still “resistance per unit area” but tangential. This reaffirms the general definition.

Why Other Options Are Wrong:
Strain measures deformation, not resistance.Pressure is external normal force per unit area applied on a surface, not the internal resistance within a solid.Modulus of elasticity is a material constant (E) relating stress to strain, not a measure of resistance by itself.Toughness is an energy absorption measure, not force per area.

Common Pitfalls:
Confusing stress (cause) with strain (effect); mixing pressure (external) with stress (internal state).

Final Answer:

stress