Frictional forces are present everywhere in our daily life. It is simply impossible to reduce them completely. Frictional forces are equally useful in some situations as they are a hindrance in others. If you look for a definition of this term you will find:
a force that resists the relative motion of objects that are in contact with each other.
There are numerous more or less detailed definitions of frictional force, but the one cited here is satisfactory for an introduction to the problem of friction.
Frictional forces exist between surfaces of two objects being in contact. Their direction is always parallel to that surface and opposite to the direction of the intended motion of an object. It is important to emphasize the word intended as frictional forces exist even if there is no motion.
Imagine that there is a box on a table. The gravitational force mg, the force pointing down towards the ground, is compensated by reaction force FR, also known as the normal force which points upward in this case, as Newton�s third law requires. We apply a force F, pointing to the right, to the box, which is measurable. We slowly increase the force and the box does not move. This means that frictional force Ff, the friction force pointing towards the left, is increasing as well and is all the time equal to F. At some value of the pulling force the box starts moving. For a moment it will accelerate and we can decrease the pulling force F to the value resulting in motion with constant velocity. Motion with constant velocity means that the net force acting on box is ZERO. This means that while the box is moving the force of smaller value is required to compensate the frictional force.
The conclusion is, there are two types of frictional forces:
static frictional force FS
kinetic frictional force FK
The frictional force in the case of solids moving against each other are defined by Equations
FS = μS mg (1)
FK = μK mg (2)
where μS and μK are coefficients of static and kinetic friction.
Their values depend on certain properties of the moving bodies and on the quality of surfaces which touch each other. This is obvious to every child. We cannot slide on our shoes on asphalt, but can easily do it on ice.
The term surface means much more than the surface of a table, floor, road or any other surface from our daily life. Later on we will discuss the frictional forces between molecules in liquid or gas and in such cases the surface of molecules plays a role in the formulas defining these frictional forces.
A very important field related to friction is motion of solid objects in the air or in water. In this case the frictional force is called drag force. In spite of a quite different name the drag force is also a frictional force and only the mechanism which creates this type of friction is very different from the one creating the friction between two solid objects.