8.1 : Dry Friction

Dry friction occurs between two solid surfaces in contact as they attempt to move relative to one another. In daily life, dry friction is encountered in various forms, such as when walking on the ground, sliding an object across a table, or rubbing hands together. Despite its ubiquity, the underlying mechanisms behind dry friction are not readily visible.

To illustrate this concept, imagine a wooden crate resting on a rough, non-uniform horizontal surface. When an external force is applied to the crate to move it horizontally, two types of forces come into play: normal and frictional forces.

In equilibrium, normal forces act perpendicular to the contact surface to balance the weight of the crate, while frictional forces act in the direction opposite to the movement of the crate. The floor exerts an uneven distribution of normal and dry frictional forces along the contact surface between the crate and the floor. This complex interplay of forces gives rise to the phenomenon of dry friction.

Upon closer examination, microscopic irregularities can be observed at the interface where the two solid surfaces meet. These irregularities create reactive forces at each contact point, contributing to frictional and normal force components. The net effect of the distributed frictional and normal forces is represented by their resultants, with the resultant normal force acting at a certain distance from the line of action of the crate's weight. This position coincides with the centroid of the normal force distribution and helps balance the tipping effect caused by the applied force.

The concept of dry friction has substantial implications for daily life. For instance, it plays a crucial role in determining the efficiency of transportation systems, as it can either facilitate or hinder the movement of vehicles. In the automotive industry, engineers must carefully consider the effects of dry friction when designing tires, brakes, and other components to ensure optimal performance and safety.

Dry friction is also an essential factor in the field of construction, as it influences the stability and durability of structures. Builders must account for the forces that arise due to dry friction when selecting materials and designing structural elements, such as foundations, walls, and roofs.