What if every surface were slippery?

Have you ever walked on ice? Whoops! It is hard to stay up.

What if the whole world were that slippery?

If you have ever stepped onto an icy sidewalk, you know the feeling: your feet slide, your arms wave, and every step is a wobble. Ice is slippery because there is very little friction between it and your shoes.

Now imagine that every surface was like that: floors, roads, desks, even your pencil. What would still work, and what would become very hard? This lab is a thought experiment, so we follow the ideas step by step.

Friction is the force that opposes relative motion between surfaces in contact. It is often treated as a nuisance, yet walking, gripping, writing and braking all depend on it.

This thought experiment sets every everyday contact to ice-like friction, with static coefficients near 0.1, and traces the consequences. The facts come from measured friction values; the consequences are reasoning, labeled as ideas.

Most things have grip. Your shoes grip the floor.

Your fingers grip your pencil. That grip is called friction.

Friction is the grip between two surfaces that touch. It always pushes against sliding. When you walk, your shoe pushes back on the floor and friction stops it from sliding, so you move forward.

How much grip you get depends on the two surfaces. Physicists measure it with a number called the coefficient of friction. Shoes on a wooden floor score about 0.9. Rubber tires on dry concrete score about 1.0. Those are grippy numbers.

OpenStax's table of measured coefficients gives static values of 0.9 for shoes on wood, 1.0 for rubber on dry concrete, 0.5 for wood on wood and 0.6 for dry steel on steel. The maximum static friction is μ_s times the normal force, so a person pressing down with their weight can push sideways with up to about 90 percent of it on a wooden floor before slipping.

Friction opposes motion, but it also lets us move; OpenStax's own example is trying to walk on ice.

Now we pretend everything is as slippery as ice.

Shoes slide. Pencils slip. Cars slide too!

In our pretend world, every surface is as slippery as ice. The number for shoes on ice is about 0.1, roughly a ninth of the grip on a wooden floor.

Follow the three paths in the lab: walking, holding a pencil and driving a car. Each card says whether it is a measured fact or an idea. The ideas are careful guesses built from how friction works; nobody has ever lived in a world like this.

Here every contact takes μ_s ≈ 0.1, the measured value for shoes on ice and for ice on ice (kinetic values 0.05 and 0.03). Any sideways force above a tenth of the normal force then causes slipping.

The scenarios trace walking, gripping and braking. Measured coefficients are labeled facts; the consequences for daily life are labeled ideas, because they are reasoned from the physics rather than observed.

To move forward, you push back on the ground.

The ground needs grip to push you forward. No grip, no push!

To walk forward, your foot pushes backward on the ground. Friction from the ground pushes your foot forward in return. If the ground can't grip, your foot just slides back and you go nowhere.

Ice is extra slippery for a surprising reason. Scientists in France found in 2019 that rubbing on ice makes a very thin film of water, much thinner than they expected, which helps things slide across it.

Static friction can rise up to μ_s·N to prevent sliding. Walking, gripping and braking all need a sideways force; when the required force exceeds μ_s·N, the surfaces slip and kinetic friction, usually smaller, takes over.

Why ice is slippery has long been debated. A 2019 CNRS and ENS study in Physical Review X measured the water film produced by friction on ice and found it far thinner and much more viscous than ordinary water.

Sometimes slippery is good! Oil helps machine parts slide.

Pencil lead is made of a slippery material too.

Sometimes people want less friction. Oil between steel parts cuts their grip from about 0.6 to about 0.05, so machines run smoothly. Pencil 'lead' is really graphite mixed with clay. Graphite is made of layers that slide over each other very easily, which is why it is also used as a dry lubricant.

So friction is not good or bad. We need lots of it under our feet and tires, and very little inside machines.

Lubrication changes friction dramatically: OpenStax lists steel on steel at μ_s 0.6 dry and 0.05 oiled, and Teflon on steel at 0.04. Graphite's carbon layers are only weakly bonded to each other and slide past one another easily, making it a solid lubricant; its best-known use is in pencil leads, where it is mixed with clay (University of Bristol).

Engineering is often about putting friction where it helps and removing it where it wastes energy.