What is the definition of friction in physics?

Friction is defined as the resistance to relative motion that occurs between two surfaces in contact with one another. This resistance arises from the microscopic interactions between the surfaces, which can include adhesive forces and surface roughness. Friction plays a crucial role in various physical phenomena, including the stopping of moving objects and the requirement of force to initiate motion.

This definition clarifies that friction is not a force that promotes motion; rather, it counteracts it. While friction can enable motion, such as a car moving forward by providing grip, it fundamentally opposes the relative motion between the surfaces. Therefore, it serves as a critical factor in understanding how motion is controlled and managed in mechanical systems. This understanding of friction is vital in applications like braking systems in vehicles and the design of machinery components, where frictional forces need to be carefully accounted for to ensure effective operation and safety.