If this article was a movie, it would probably be titled Friction: Origin Story and it would portray how a good guy developed into a notorious villain.
In many ways, the force of friction feels like a villain of some kind in the study of mechanics. When every other force seems to be doing some credible work, the force of friction is just there to oppose them.
But as we shall soon find out, when you take a deeper look, you will realize that friction comes from a rough background – no pun intended!
What is the force of friction?
Friction is the force that opposes relative motion between bodies that are in contact with one another.
There are many forms of frictional forces depending on the type of motion, mode of contact, or the state of matter of the substance.
For example, when a solid body moves through a fluid (liquid or gas), the force of friction associated with that motion is known as the Drag Force. This is different from the force of friction that arises when two solid bodies are sliding past one another.
For this article, we shall consider the latter, i.e. the frictional force between two solid bodies in contact.
Daily experiences with friction
Think of what happens when two solids slide past one another – like a crate sliding down the flow. First, you’ll notice that a larger force is used to set the crate in motion than the force used to maintain it in motion.
This discrepancy is not because of inertia, which is the property of the body to resist change in motion, but rather it has to do with the force of friction. In fact, there are 5 fundamental differences between inertia and the force of friction.
Secondly, you’ll notice that the crate will make a screeching noise and even leave some marks on the floor as it slides. This implies that the two surfaces are interacting fiercely at the point of contact, releasing sound and heat energy as well as leaving deformations on each other. Friction is a gruesome force.
This is where the conventional experience of frictional force typically stops. However, some have taken a deeper look into the workings of the force of friction and come up with some interesting things.
Laws of friction
So far friction has been easy to understand right? But as they say, “It’s a bit more complicated than that.”
When French physicist, Guillaume Amontons, closely studied the properties of friction of rectangular blocks sliding over a flat surface – he was surprised at some of his results.
His conclusion now constitutes what we know as the classical laws of friction.
- The force of friction does not depend on the weight of a block. Rather friction is proportional to the “normal force” – or the force that squeezes the surfaces together. More on the normal force here.
- The amount of friction does not depend on the “apparent” area of contact. A small block sliding over a surface experiences as much frictional force as a large block of the same weight.
- The force of friction does not depend on the velocity once the motion starts. This was discovered later by fellow French physicist, Charles-Augustin de Coulomb.
Since then, modern physicists have been on a quest to explain the erratic behavior of friction at the atomic level.
As it turns out, the force of friction is one tough nut to crack. This villain is going to take a “Sherlock Holmes”-standard detective to crack him. So physicists rolled up their sleeves, put on their mean faces, and started poking around hoping to unravel friction’s mysterious behaviors.
The force of friction at the atomic level
Tribology is the study of friction. It is like the special division of the FBI or Interpol responsible for investigating our mean villain – friction. The best way to understand your enemy is to study their origin, and in the case of friction, that origin is down at the atomic scale. So let’s dive in!
True Area vs. Apparent Area
At the atomic level, even the finest polished surfaces are far from flat. There are always ridges, mountains, and valleys. Therefore, when two surfaces come in contact with one another, the actual or true area in contact is significantly less than the apparent area in contact.
This explains why the force of friction is independent of the “apparent” area in contact.
Plastic deformations
Since the true area in contact is very small, the points of contact between surfaces are under intense pressure. In fact, they are so pressed together that they deform and become “cold-welded” together.
Thus when a body is forced to move over the other, these tiny welds continually rupture and reform. These “stick and slip” events is what are responsible for the noises that dry surfaces make when sliding across one another.
Sliding speed, sliding direction, temperature, speed of sound?
Like I said earlier, everything is always, “a bit more complicated than that”. And friction, unsurprisingly, is no exception.
At the subatomic level, nanotribologists (scientists studying friction at the atomic level) keep discovering how friction depends on many other factors.
It seems like the more we learn about the origin of friction, the more mysterious it becomes. Sometimes, that’s what makes villains interesting. The question remains on whether our “Sherlock Holmes detectives” will live up to the challenge.
Conclusion
Whilst the force of friction might seem confusing and mysterious at the atomic level. Our molecular understanding of friction has incredible potential applications in the fields of nanotechnology, superfluidity, and biotechnology.
Leading scientists in the field believe this could save tons of energy and machine wear, improve efficiency, develop new drug delivery techniques, and so forth.
If you enjoyed this article, you might like why friction force exists.
