What is friction?

shutterstock

The Most Common Origin of Friction

Friction exists because most surfaces really look like this under a microscope.

Note the hills, the crests and the valleys. These are called asperities, or material deformations. They occur on rough surfaces, and even flat surfaces might have minor irregularities on it.

Now imagine trying to rub them against each other. Feel, for a moment, in your mind’s eye the opposition you are likely to encounter as they grind against each other.

That grinding is the most common origin for friction, both static (when your object is still and you’re trying to get it to move) and kinetic (when your object is already moving, and you need to keep it moving).

It is why the friction that wheels encounter while rolling (called rolling friction) is much lower than other types of friction, and why tyres have deep grooves in them. It is also why friction tends to decrease the flatter and more uniform your surface gets.

Friction for Flat Surfaces

But friction can still exist for perfectly flat surfaces, where asperities of the form above are no longer present.

Perfectly flat surfaces bond to each other - in other words, they try to stick to each other. This is called adhesion, and can be caused by any number of reasons:

• van der Waals forces, a class of forces where attraction between electric dipoles on the surface of the material cause them to be attracted. In such cases, the materials are said have been dispersively adhered.
• chemical bonding, where the atoms of the two materials exchange electrons in some way and become chemiially attached to each. For instance, hydrogen in one material may form tenuous bounds with strongly electronegative elements in the other.

These forces are typically weak enough that they are easily broken once you lift the object - but not so weak that overcoming the attraction by dragging the object doesn’t pose a problem.

Finally, a Definition

Friction, then, is not so much a force as it is the sum of very tiny forces and irregularities along the edge of surfaces.

It is possible to model it as a true force - and indeed, most high school textbooks do just that - but it is important to recognise that that is just a convenient abstraction. Friction is to forces what Santa Claus is to a jolly man in a fat suit: window dressing on top of a very complex reality.