Friction, Types of Friction | Physics@TutorVista.com
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Friction is unavoidable in our day-to-day lives. Man keeps trying so many techniques to reduce friction in any process, but he also realises that without friction none of his work would have been possible. Life would have been impossible without the aid of friction. Yet friction is termed as an evil. Without friction one can't walk, write, sit on a chair without slipping off or for that matter, even properly hold on to his or her morning cup of tea.
A baseball player rubs against the ground while sliding into the home plate, a sports  car creates friction when it rolls across the floor and the wheels rubbing against the floor causes the car to slow down and stop. In both of these cases, it's the friction which makes the moving body slow down and stop. Some surfaces with many grooves and bumps can cause a lot of friction while a surface with very little grooves and bumps can cause very little or no friction at all. In this section, we will discuss and learn more about this topic.
 
Rolling friction
Sliding friction
1. Kinetic Friction: When two bodies that are in contact with each other and move rubbing the surfaces that are in contact, the friction existing between them is called kinetic friction. The direction of the force is such that the relative slipping is opposed by the retarding force (friction).
Magnitude of Kinetic Friction
$F_{k} = U_{n}N$
Where,
Fk = magnitude of kinetic friction
N = Normal reaction
Uk = coefficient of kinetic friction
The coefficient of friction does not depend upon the speed of the sliding bodies. If the surfaces are smooth then it will be small, and it will be large if the surface is rough.
2. Static Friction: The opposing force which comes into play when an object does not move over another object, even when the force is applied to make it move is called Static Friction. Hence it can also be defined as a force which comes into play when two bodies are not moving relative to each other. Static friction prevents objects from sliding or rolling over each other.
For example, when we push a heavy object, because of the friction the object is unable to move through the surface. Then we apply some force on the object. Once it moves through the surface, it is very easy to move further.
When we were unable to move the object, it was static friction. When it was moving we were overcoming the kinetic friction, which is found to be less than static friction.
$F_{s} = F_{max}= U_{s}N$
where,
Us is the Coefficient of Static friction
N is the Normal reactionStatic friction is greater in magnitude than kinetic friction in any situation.
3. Dry Friction: Dry friction between two objects in mechanical contact is normally assumed to be the influence of dry friction force. These forces are generating in the tangential direction of the plane of contact and both prevents the movements between the surfaces.
Dry friction is basically subdivided into two types:
Static friction between static objects
Kinetic friction between moving objects
4. Fluid Friction: This is a friction which comes into play when layers of any viscous liquid move over each other. Hence the name is Fluid friction. Viscosity is also called as thickness. Also, Movement is inversely proportional to the viscosity. For example, water is thin hence lower viscosity than honey.
5. Lubricated Friction: There are conditions when a layer of liquid exists between two solid surfaces, in such a situation, the friction which exists is called lubricated friction. Lubrication is a technique by which a viscous liquid is introduced in between two moving surfaces which aids for motion (smooth) and also prevents wear and tear of the surfaces hence increasing their life span.
6. Skin Friction: The force which resists the movement of a solid body through a liquid is called skin friction. This directly depends on the area of contact of the body with the fluid.
There are various ways to reduce it:
Improving the shape of the object
Reducing the surface area as far as possible
7. Rolling Friction: When a round object rolls over some surface then rolling friction comes into play. It is dependent on various factors like deformation of surface and object, diameter of the wheel, speed etc.
$F = C_{rr}N$
Where,
Crr = coefficient of rolling friction
8. Sliding Friction: When two surfaces slide over each, then the friction which comes into play is called Sliding Friction.