What defines a rigid body in mechanics?

A rigid body in mechanics is defined as one that does not deform when subjected to external forces or loads. This assumption of rigidity simplifies the analysis of structures and mechanical systems, allowing engineers to apply principles such as equilibrium, motion, and energy conservation without needing to account for internal deformations.

In engineering mechanics, this concept enables a clear and straightforward approach to problem-solving, focusing on the overall system behavior rather than complex interactions resulting from material deformation. While all real materials do exhibit some degree of deformation when loads are applied, a rigid body is an idealization that is particularly useful in many engineering analyses where the deformations are negligible compared to the overall motion and configuration of the body.

The other choices incorporate concepts that do not align with the definition of a rigid body. For example, a body that deforms when loaded indicates elasticity, which directly contradicts the rigidity assumption, while a type of fluid under pressure pertains to fluid mechanics rather than the concept of rigidity in solids. The mention of a body subjected to high temperature brings in thermodynamic effects, which may cause deformation but does not specifically address the criteria for defining rigidity in a mechanical context.