What does stiffness in a material refer to?

Stiffness in a material refers to its resistance to deformation when subjected to an applied load. It quantifies how much a material will deform—specifically bend or stretch—under a certain force. Stiffness is crucial in engineering and design, as it affects how structures behave under stress and determines the ability of materials to maintain their shape and integrity under load.

When a force is applied to a stiff material, it will not easily change shape; thus, it exhibits a high degree of rigidity. This property is particularly important in applications that require materials to support loads without significant deflection, such as beams in buildings or components in machinery.

The other options focus on different material properties that do not directly define stiffness. For example, the notion of strength and ductility relates to a material's ability to withstand loads before failing and its capacity to deform without breaking, respectively. Conductivity relates to how well a material can transfer heat, while thermal resistance pertains to a material's ability to resist heat flow. Each of these properties contributes to a material's overall performance but does not directly indicate its stiffness.