What is the definition of "friction" in mechanical terms?

Friction is defined as the resistance that one surface or object encounters when moving over another. This resistance arises from the interactions between the surfaces in contact, which can include microscopic roughness and adhesive forces that prevent smooth motion. In mechanical terms, friction is crucial as it affects how objects move and interact with one another. It plays a significant role in various applications, such as brakes in vehicles, where friction helps slow down or stop motion.

Understanding friction is essential in engineering, as it directly influences the efficiency and performance of machines and structures. For example, while friction is necessary for gripping and controlling motion, it can also lead to energy loss in the form of heat, which is what distinguishes it from concepts like acceleration or inertia.