What is the internal resistance to an applied external force?

The term that best describes the internal resistance to an applied external force is stress. In materials science and engineering, stress is defined as the force exerted per unit area within materials. It arises from externally applied forces, uneven heating, or permanent deformation, and it is a measure of the internal reaction of the material trying to maintain its original shape and size when subjected to external loads.

Stress is typically measured in Pascals (Pa) and can be calculated using the formula:

Stress = Force / Area

where the force is the applied external load, and the area is the cross-sectional area where the force is applied. This internal resistance allows materials to withstand various loads before failing or deforming permanently.

The other terms mentioned have distinct definitions that do not directly capture the concept of internal resistance. Load refers to the external force applied to a structure, ultimate strength denotes the maximum stress a material can withstand before failure, and strain represents the deformation or displacement per unit length in a material due to applied stress. These concepts are interrelated but do not specifically define the internal resistance to external forces in the same way that stress does.