Does friction always oppose motion?

Does friction always oppose motion?

A recent discussion on twitter, started by the question below, prompted me to write this blog.

https://twitter.com/i/status/1263828037189206017

To many of us the diagram looks wrong. If the runner is moving forwards, shouldn't the force arrow of friction be pointing backwards?  After all,  friction is a force that always opposes motion, isn't it?  Well, the answer is both yes, and no. It depends on what motion you are talking about. let me explain.

Consider a sprinter who is pushing off a starting block. If we look at the forces involved we see that the sprinter is applying a force on the starting block backwards and downwards. The starting block applies an equal and opposite force on the sprinter (Newton's Third Law). Importantly in this example, the force which is in the direction of motion is the force applied by the starting blocks on the sprinter.

The forces acting between a sprinter and starting block

Now consider a person running upright on a flat track.  Similarly to the sprinter above, the runner's legs are applying a force backwards. We can be sure of this because the at the point of contact between the runners shoe and the track, the shoe moves backwards.  The track applies an equal and opposite force on the sprinter (Newton's Third Law), and this is because of friction.  This is where friction opposes motion.  Not the overall motion of the runner, but the motion of the runner's foot when it is in contact with the track.  We can then say that the force applied to the runner in the forwards direction is the friction force.

The Forces acting between a person running upright on a flat track and the track

Now consider a car moving forwards, with the car tyre in contact with a road surface.  Assuming that the car is front wheel drive, the tyres are applying a force on the road surface backwards. We can be sure of this because the at the point of contact between the tyre and the road surface, the tyre moves backwards.  The road surface applies an equal and opposite force on the car (Newton's Third Law), and this is because of friction.  This again is where friction opposes motion.  Not the overall motion of the car, but the motion of the car tyre when it is in contact with the road surface.  We can then say that the force applied to the car in the forwards direction is the friction force.

The Forces acting between a car tyre and a road surface

So, Does friction always oppose motion? Only when two surfaces are in contact with each other.  All of a runner and all of a car are not in contact with another surface (unless we consider air to be a surface), but importantly, a part of a runner or a car is in contact with another surface.  When we inspect the forces and motion at these points of contact we find a shoe and a tyre moving backwards, and applying a force in the backwards direction.  We also find friction between the surfaces applying a force in the forwards direction.

I hope you find this blog post useful.

Dan (@dan_braith)