Blue Sky Science: How does friction work?

Daniel Bukhman


How does friction work?

Friction is a force that resists sliding motion between contacting surfaces.

A bike, for example, has many instances of friction. The brake pads and the wheel rims have friction when we brake. There is friction between the handlebar and the hands that allows for a successful gripping. The bike tire and the road have some friction so that we can actually roll the bike wheels safely without sliding.

There are two major physical mechanisms that control friction. The first is how sticky the sliding surfaces are, and the second one is how much plowing the motion causes by sliding those two surfaces against each other.

If you try to slide a small wooden box over a smooth table, it will be relatively effortless. You would still feel some amount of resistance, but not much. That resistance is friction.

If you try the same type of experiment on a table covered with double-sided sticky tape, then the resistance increases, friction is higher and it’s more difficult to move the box over that surface. This is an example of the first, stickiness mechanism that controls friction.

When thinking about the second, plowing mechanism, imagine yourself shoveling snow. When you’re removing snow, the taller the snow piles, the higher the forces that are needed for you to remove it.

About 500 years ago, Leonardo da Vinci conducted the first systematic set of experiments on friction moving boxes over a wooden block. He tried to see how friction changes as the surface area or the mass of the slider changes.

He found that friction is independent of contact area or surface area of the slider; in other words, whether you slide a box on its larger face or smaller face, the resistance is the same.
He also noticed that if he doubles the mass of the slider, then the resistance or friction also doubles. So we know friction is actually proportional to the mass of the slider.

Those are considered the classical laws of friction.

Today, we conduct research to control friction and its consequences. Some examples of ongoing research include finding nearly frictionless and wear-free surfaces; active tuning of friction, and friction in biological systems such as animals, plants and human body parts.