Final answer:
The rigid bar AD described is a mechanical structure; a part of a system designed to support or handle force and maintain shape, commonly used in engineering applications.
Step-by-step explanation:
The rigid bar AD mentioned in the question is a type of mechanical structure. This term typically refers to a rigid structure that is part of a system designed to support or transmit force, maintain a shape, or hold components in place. In engineering, such structures are made from materials like steel and are designed to withstand the physical stresses and loads that they will encounter in their environment. Examples of mechanical structures include beams, frames, and trusses used in buildings, bridges, and various types of machinery.