### Feel the force around you – Frictional force Part III

This is the third article in a series in order to explain the force of friction. Possibly, this is the most interesting article beside all related, which links could be find at the end of this one. This article gives an answer to the question why friction does exist. In this article we will skip all mathematical and physical background and focus on very simple examples.

Friction does exist because the surface is not 100% smooth, and there is no such an object (including living beans) in nature with 100% smooth surface. That is the reason why friction does exist. But something should be clear, the frictional force is a reaction force, which means that it exist only when one object has tendencies to move, interacting with some other object (when two objects are connected). On bellow picture we see two connected objects. Both objects has saw tooth surface. When these two objects are connected, nothing happens, but when the above object has tendency to move, driven by some external force, frictional force is generated in order to prevent motion and it acts in the opposite direction in order to nullify the effects of the applied external force. If an external force is applied to the right side of the above object, left parts of saw-toothed surfaces are involved in the frictional force reaction and vice versa. Objects with saw-toothed surfaces are just an example for the sake of simplicity.

 Surface
In reality it is hard to see rough surface. For example, our perception saying to us that a glass of our window is perfectly smooth, but it is not, because we are not able to see on a microscopic level. To see all that roughness we have to zoom in. Without roughness world will never look like it looks now. Roughness is the perfect design of nature or creator (depends on your opinion, whatever it is, and it is your choice). For example: We are all born naked, we were not born with shoes. Our body is perfectly designed for controlling stable movement and interactions with surrounding space. Pay attention that foots are equipped with footprints and our fingers with fingerprints. Why? The answer is simple, without roughness on feet and fingers we will never be able to provide controlled walking or even to carry a glass of water. Without footprints, it would be more sliding over the street instead of walking. Footprints are little roughness on our body which interacts with other rough surfaces, in order to provide stable motion and interaction (without sliding). So, if you ask yourself: Why we do not have prints on our head? The simplest answer is: because the head is not intended for walking and motion providing, it is designed for something else, thinking. Now, think about your shoes…

 From left to right: Footprint and fingerprint

Another perfect example is a perfectly designed machine called the fly. The fly has possibility to land on any surface, to land on the glass on your windows, to land on the ceiling of your room. Why? Because there is no 100% smooth surface, all surfaces have some percentage of roughened. The fly is similarly designed as human beings, with obvious difference, it does not have footprints and fingerprints. Instead, it has claws on the legs. With claws and imperfect smooth surface, the fly is able to stand and walk (controlled walk) on any place from its surrounding. If the glass of our windows is perfectly smooth, without any roughness, the fly will never be able to stand or even walk, it would be sliding.

 The Fly

And last for this article, when you walk on ice within your shoes think about frictions. In the upcoming articles we will see that coefficient of friction is additional parameter regarding frictional force. There are complete sets of frictional coefficients, depending on materials of two or more connected surfaces. Believe it or not, everything can be seen from mathematical equations...

 The shoes

Related articles:
Feel the force around you – Normal force
Normal force - Real catalog examples and calculations
Feel the force around you – Frictional force Part I
Feel the force around you – Frictional force Part II
Feel the force around you – Frictional force Part IV