What is Friction?
Friction is the resistance to motion of one object relative to another. It is not a fundamental force like gravity or the electromagnetic force, but rather occurs because all surfaces have roughness. Even surfaces that seem very smooth still have roughness, although you might have to view them at the molecular level to observe the roughness. Roughness occurs in various ways. For rougher surfaces, like two pieces of wood, the physical hills and valleys of the surface interlock to some extent and provide friction. For other surfaces that are smooth down to the molecular level friction occurs due to the electromagnetic attraction between charged particles in touching surfaces.
Liquids have friction as the moving layers of fluid move past each other. Thicker liquids have more friction than less thick. Think about molasses vs. water. Likewise gasses have friction. Think about a skydiver falling through air – what we call “air resistance” is friction
Two Types of Friction
There are two types of friction: static and kinetic. Static friction is between two objects that are not moving relative to each other and prevents objects from starting to move. Static friction is what keeps an object resting on an incline from moving. Kinetic friction refers to the friction that acts on object moving against each other. Static friction is generally higher than kinetic friction.
Coefficient of Friction
The main components of friction are the amount of perpendicular force applied between two objects and the coefficient of friction.
There is no formula for the coefficient of friction – rather it is determined through experiments. Here’s a chart of some friction coefficients:
Friction is Not Dependent on Surface Area
A fascinating property of friction is that it is not dependent on the surface area in contact between two objects. This seems counter-intuitive. Here’s why this is the case: “Although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together.” Source.
That friction is not dependent on the surface area of objects was first observed by Leonardo da Vinci. He was a pretty smart dude. He never published his works on fiction (as was true for most of his discoveries) and had to be re-discovered by others later on.
Related IFOD (that has a surprisingly high number of views): Density!
I believe I read somewhere that after the Industrial Revolution made blue jeans widely available, denim on denim friction caused the nearly exponential growth of human populations.