What is Friction?

Stuff keeps going until something stops it. That’s the basic idea of Newton’s First Law, right? A ball hitting a wall or something. Yet, we also know that if we push a skateboard down the sidewalk, it’s gonna stop eventually even if it never hits anything. So, why?

Friction.

The sciencey explanation is that friction is caused by microwelds, or small bumps on the surface of two or more things sliding together.

Whether you realize it or not, virtually everything you think is smooth is only smooth from your perspective. Zoom in closer and virtually everything is actually quite bumpy!

I mean, basically smooth is just a matter of scale.

When two surfaces touch, like wood and sandpaper, their bumbps and dips touch too which makes them stick together. That sticking is called microwelds, and it takes a force to break them.

The amount of friction depends on two things, really: 1) just how bumpy one or both surfaces are, and 2) how much pressure is between the two.

The rougher the surface, the more bumps. The more bumps, the more microwelds. The more microwelds, the more friction!

Or

the more pressure, or force, pushing two objects together, the more bumps come in contact, and so-on, and so-on.

Which brings us to the types of friction.

Ever try to push something across the room and it doesn’t want to budge? No one is pushing it back at you, but you still need enough force to overcome the friction and break those sticky microwelds. The force of friction cancels out your push and there’s an overall net force of zero.

Push harder and you can break through the friction. As soon as you let up, the box or couch or whatever is going to stop moving because as it moves it is constantly making and breaking microwelds.

So far, we’ve really only used friction as something that’s bad or causes some kind of disadvantage, but the truth is that without friction there would be no skateboarding, rollerblading, driving, or anything else like that.

Imagine you’re skateboarding down the street. The parts of your wheels that are touching the ground are experiencing static friction. The forward movement of the round wheel makes it so a new part of the wheel touches the surfaces as it rolls, creating static friction each time which keeps the wheels from just sliding. This idea tends to be referred to in everyday life as traction. Think about trying to drive in snow or on an icy road. Without friction, the wheels just spin in place! You might put sand down to make more traction for your tires.