Physics

Friction occurs because of interactions between the surface layers of two solid materials. The exact nature of the interaction depends on the microscopic properties of each material, such as its chemical composition and crystalline structure. 

Friction is a complex phenomenon that arises from electrostatic forces between electrons in atoms and molecules; van der Waals forces arising from changes in electron density, pressure and shear stress; changes in the normal contact forces between layers of atoms or molecules as they slip past one another; and other effects.

The frictional force is always proportional to tangential force by the friction coefficient. A body’s static frictional properties can be changed by heating, which causes the atoms and molecules in a material to expand; therefore friction increases with temperature.

Normal force is an object’s weight, whereas static frictional force is independent of any weight components. A sliding body or surface exerts tangential forces that may be resolved into two components: normal forces, perpendicular to the contact surface, and frictional forces parallel to it. The maximum friction is then reached when a component of the frictional force acts along that normal line; this determines a static frictional torque.

The friction between materials depends on properties such as their composition, temperature, pressure and cleanliness (smoothness or roughness). The frictional force can be calculated by considering the materials to have μ-scale rough surfaces.

A body with a higher coefficient of friction (μ) will accelerate more slowly in response to the same torque as another body having a lower μ value. This is because the tangential force (the component of the total force parallel to the contact surface) must be greater for low μ, and thus a lower tangential acceleration results. The graph at right helps determine when friction is dominant over inertia.

The main advantages of friction are:

1. Prevention of slipping
2. Prevention of sliding down slopes (friction holds us on the road)
3. The generation of heat allows “thermal energy” use in certain industries such as steel production and processing, chemicals, glass manufacturing etc. Heat results from friction when contact pressure is great enough to overcome thermal resistance.
4. Friction creates an overall downward force, which prevents the crate from sliding down and hitting other objects.
5. Friction makes the system stable. It resists any small disturbances in position of the crate so it can settle at equilibrium position without falling off.

The main disadvantages of friction are:

1. Friction gives rise to wear, which is undesirable in most mechanisms and machine parts. It can become extreme enough to damage components or cause failure at a certain point. For example, engines have cylinder walls that rub against the top and bottom of their cylinders (most often in the form of piston rings), and when this rubbing is enough to overcome the strength of engine material, cylinders can wear out. Friction may also be a source of noise in mechanical components.
2. When friction results from one object sliding over another (e.g., shoes on ice), it produces heat. This is a waste of energy and hence thermodynamic loss, which would be avoided if the two surfaces could repel each other.

Friction is also important in many biological systems, such as muscle contraction where actin-myosin interactions are used to generate mechanical force and “wear down” over time.

Humans may experience friction as a consequence of contact between the soles of their feet and the ground, when they skate forward on a slippery or rough surface.