Question

What is friction ?

Answer 1

Hint:Friction is a type of contact force which comes in picture when two bodies are kept in contact. It has certain advantages as well as disadvantages in our daily life.Friction is an intermolecular force between two bodies that opposes the relative motion or tendency of relative between two surfaces in contact.

Complete answer:
The magnitude of the frictional force depends on the nature of surfaces in contact. The frictional force is usually larger on the rough and smaller on the smooth surfaces.Friction is caused by the irregularities on the two surfaces in contact.Friction is always parallel to the surface in contact.

There are many kinds of friction that exist in different media:
-Static friction: It exists when two surfaces try to move across each other but not enough force is applied to cause motion.
-Sliding friction: It exists when two surfaces slide across each other in contact.
-Rolling friction: It exists when one object rolls over another object.
-Viscous friction: It exists when two objects move through water or other liquids. It is also known as fluid friction.

Static friction (${f_s}$): It is the force exerted on an object at rest that prevents the object from sliding.
-The direction of static friction is opposite to the applied force. Also, it acts in a direction opposite to the direction in which an object tends to move.
-The maximum value of statics friction is called limiting friction. It is the minimum amount of force that must be applied to make a stationary body start moving.
-The law of statics friction is \[{{f}_{s}}\,\le \,{{\mu }_{s}}N\], where \[{{\mu }_{s}}\]= coefficient of static friction, which depends only on the nature of surfaces; \[N\]= normal force (or normal reaction)
-Limiting (maximum) value of static friction is given by, \[{{f}_{L}}\,=\,{{\mu }_{s}}N\]. If the applied force F exceeds \[{{f}_{L}}\] then the body begins to slide on the surface.

Sliding friction (or kinetic friction) (${f_k}$): It is the force exerted on an object in motion that opposes the motion of the object as it slides on another object.
-Sliding or kinetic friction is smaller than the limiting value of static friction. This is because it takes more force to break the interlocking between the two surfaces than it does to keep them sliding once they are already moving.
-The law of statics friction is \[{{f}_{k}}\,\le \,{{\mu }_{s}}N\], where \[{{\mu }_{k}}\]= coefficient of static friction, which depends only on the nature of surfaces; \[N\]= normal force (or normal reaction)

Rolling friction: When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction. Rolling reduces the friction significantly. Hence sliding is replaced in most machines by rolling by the use of ball bearings

Viscous friction (Fluid friction): The force of friction exerted by the fluids on the objects moving through them is called fluid friction.Fluid friction depends on the speed of the object. The faster an object moves in a fluid, the greater is the fluid friction acting on it.It also depends on the nature of fluid. For example, an object moving with certain speed will experience more friction in honey than it experiences in water.

Note:Friction is necessary evil: It means it has advantages as well as disadvantages. In other words, friction is not desirable but without friction we cannot think of survival.A significant amount of energy is wasted in the form of heat to overcome the force of friction. Friction causes a lot of wear and tear in the moving parts of the machine due to which its maintenance is required. The efficiency of the machine decreases due to presence of force of friction.