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

**This is the first article in a series intended to explain the force of friction. If somebody asks you: what is the force of friction? Just give a simple answer, it is a force! Maybe it sounds crazy, but it is a 100% true answer. Let’s start thinking in equations. If we do have force on the left side of an equation that also mean that we do have a force on the right side of the equation. It is not possible to have force on the left and frogs on the right, because force and frogs are not the same things by nature. Basically,**

*what is a force of friction?*is a stupid question. The better question is: which force is the force of friction?
That is a very good question. We already started a discussion about the normal force included in the frictional force calculations. Anyway, even
if a normal force is used to calculate the maximum value
of the frictional force, there are a lot of differences between normal
and frictional force, and also similarities. The frictional force is a reaction force (acts in the opposite direction) as it is a normal force, but with different behavior. Frictional force has different behavior depending on the situation, but
we will see that behavior also depends on the geometrical shape of the
object. In some cases it works against motion in some cases it provides the motion.
Comparing to other natural occurrences like temperature, friction is a
natural phenomena. Mathematically spiking temperature is a continuous
function without interrupts, like it is presented in the following
equations:

Continuous function in general. |

On the other side frictional force (vector
F

_{f}) has interrupt in point where applied external force (vector F_{E}) overcomes the normal force (vector F_{N}) multiplied by a frictional force coefficient (C_{f}), as it is presented in the following equations:Motion descriptions relate to the sliding motion of the cubical shaped objects |

The frictional force interrupted function |

By equations and the above picture, frictional force
contains two domains: static friction domain and kinetic friction domain. In
some cases static friction domain works against motion in some cases it provides
the motion.
We will see that it depends on the geometrical shape of the object.
For example: for cubes (and similarly shaped objects) it works against the motion.
In that case it is important for external forces to overcome the maximum frictional force.
On the other side, for rounded objects like wheels static friction domain provides the motion.
Unlike static friction, kinetic friction always act against the motion.
Now, let’s answer the question: which force is the frictional force? Well, the frictional force
is a reactional force
which means that it is equal to the applied external force, it acts by Third, Newton
Law and it is linear, in case when it belongs to the static friction domain. To
calculate maximum static frictional resistance (force) it is necessary first to
calculate a normal force, which means that first we need to know the total weight of the mobile robot including incline angle. In the next articles we will see
that maximum normal force depends on inclines, which means that normal force is
not a constant. When an
applied external force acts in kinetic frictional domain, the frictional force
is also a reaction
force, but with less intensity which leads to the motion. In
general, Frictional force is reactional force, like normal force is,
in case when applied external force is greater than zero.

Feel the force around you – Normal force

Normal force - Real catalog examples and calculations

Feel the force around you – Frictional force Part II

Feel the force around you – Frictional force Part III

Feel the force around you – Frictional force Part IV

Feel the force around you – Frictional force Part V

Programmable autonomous vehicles – Fundamentals, Part I

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