Types of friction

The frictional force is that force responsible for wear and tear of two bodies. 

When two bodies in contact, move relative to each other then there exists a force which has a tendency to oppose that movement which is called frictional force. 

Friction plays an important part in many everyday processes. For instance, when two objects rub together, the friction causes some of the energy of motion to be converted into heat. This is why rubbing two sticks together will eventually produce a fire.

Friction is also responsible for the wear and tear on bike gears and other mechanical parts. That's why lubricants, or liquids, are often used to reduce the friction and wear and tear between moving parts. 

 There are in general  4 different types of friction.

1. Static Friction
2. Kinetic Friction
3. Rolling Friction
4. Fluid Friction

Static Friction :

• When a force is applied to an object but it does not cause it to move

Static friction comes into play when a body is forced to move along a surface but the movement does not start. 
This is the maximum force that must be overcome before a body will just start to move over another body. 
The magnitude of static friction remains equal to the applied external force and the direction is always opposite to the direction of motion. 
The magnitude of static friction depends upon μs (coefficient of static friction) and N (net normal reaction of the body).
 Example: Pushing on a wall

Kinetic Friction :

• Occurs when solid surface sliding over each other

 Kinetic friction denoted as μk comes into play when a body just starts moving along a surface. When the externally applied force is sufficient to move a body along a surface then the force which opposes this motion is called as kinetic frictional force.

This is the maximum force that must be overcome before a body will just be in uniform motion over another body.                                  

Magnitude of kinetic frictional force f_k = μk N

Where μk is coefficient of kinetic frictional force and N is the net normal reaction on the body. The magnitude of kinetic frictional force is always less than magnitude of static frictional force. When value of applied net external force F is more than f_k then body moves with a net acceleration and when these forces are equal then body moves with a constant velocity.

Example : Falling on the pavement

Rolling Friction :

•  Occurs when an object rolls over another 

Rolling frictional force is a force that slows down the motion of a rolling object. Basically it is a combination of various types of frictional forces at point of contact of wheel and ground or surface.When a hard object moves along a hard surface then static and molecular friction force retards its motion. When soft object moves over a hard surface then its distortion makes it slow down. 

Example: Riding a motorcycle, Skateboard 

Fluid Friction :

• Occurs when a object moves through a fluid 

When a body moves in a fluid or in air then there exists a resistive force which slows down the motion of the body, known as fluid frictional force. A freely falling skydiver feels a drag force due to air which acts in the upward direction or in a direction opposite to skydiver’s motion. The magnitude of this drag force increases with increment in the downward velocity of skydiver. At a particular point of time the value of this drag force becomes equal to the driving force and skydiver falls with a constant velocity.

Example : Skydiving, Swimming