Friction, an Opposing Force

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This is a basic physics tutorial that is targeted at GCSE (grade 9 and grade 10) standard reviewing friction forces. The video for this written tutorial is on its way.

What is friction?

Friction is a type of contact or applied force, but it is slightly different to those described in the table and block example in the previous tutorial. This is because friction is not an applied force; it is a reaction force. It can also be described as an 'opposing' force. That is, it is only in existence in reaction to another force.

When does friction occur?

It occurs whenever a solid rubs against another solid, a liquid or a gas. This tutorial will focus on the friction that occurs between two solids. The following tutorial will review examples of friction between a solid and a gas.

Friction between two solids

Take your hands and rub them against each other really fast. That heat that develops is a result of the friction between the skin on your two hands; this can be useful when you're feeling cold. Let's look another more everyday example : walking.

When you walk, friction acts in many ways to prevent you from slipping. Initially it provides resistance so that you can put your foot down safely (#1 below). Then as you push backwards against the ground, it pushes you forward, allowing you to walk (#3 below).

If there was no friction (or much less than you expected), your foot would slip forward, and you would end up on your back. A classic example of this is slipping on a banana peel, or on ice.

If you wear socks on polished timber floorboards, there is less friction than you're instinctively used to. If you're not careful, you will slip, most likely when pushing off the ground. This can result in you ending up on the ground on your stomach.

Another example of friction between two solids is the friction between car or bike tyres and the road. Let's take a look now at air resistance and terminal velocity.