How does friction work?

We all know what friction is. It's what happens when two surfaces rub together. Friction is the enemy of your car's engine and it plays a role in making mechanical things wear out. But, what the heck causes friction? If we go back to the classical definition: it's the force that causes resistance in moving one surface over another.

One way that we experience the force of friction is when we detect heat from rubbing two surfaces together. Everyone is familiar with rubbing your hands together when it's cold to heat them up. That friction force is converted to heat. In other words kinetic energy is converted to thermal energy.

The first thing that physicists realized is that the force of friction is directly proportional to the applied load. In other words the heavier the objects are or the harder they're rubbed together the more force that's required to move them. That makes sense.

The next thing is that the force of friction is independent of the area of contact, and the kinetic force is independent of the sliding or rubbing velocity. That's cool!

One thing that became obvious is that the smoother that the two surfaces in contact are the easier it is to slide them on each other. Also, the more precisely flat they are, the easier it is. The question is: why is this the case? Even though it seems intuitive, it wasn't until microscopes and even electron microscopes came along that the real reason was discovered. What was discovered is that even though a surface appears smooth, there could be lots of imperfections. Looking at a surface under high magnification reveals hills and valleys as well as holes and protrusions. All of these imperfections cause friction. When you get down to the atomic level you see many more imperfections. This is where the friction happens. Friction happens at the atomic level where atomic outer electron shells slide against each other.

One of the ways to eliminate or lessen friction is to make the surfaces as smooth as possible. That's why a car engine's bearings are made to high tolerances. The other method is to introduce grease or oil to substitute solid friction with fluid friction. This is called lubrication, and it's the main way to lessen friction. Fluid friction depends upon the viscosity and the ability to keep two metallic surfaces apart. A lot of work has gone into this aspect of engine lubrication to create consistent lubrication over a wide temperature range and under severe workload. It's the reason that you can drive for many thousands of miles these days without changing oil.

Not all friction is bad. Think about how the breaks on your car work. If it weren't for friction, you couldn't stop. At the same time, break friction causes the break parts (the pads and the rotors) to heat up and this heat must be dissipated. That's where physics come into play. Car designers have come up with ingenious ways to dissipate the heat of breaking so you don't have to worry about it. Amazing, isn't it?

Thanks for reading.