Friction :- It is the force that opposes the motion of an object.

Factors affecting friction : -
(i) it is caused by the irregularities on the two surfaces in contact.
(ii) When the surfaces on any two objects are in contact, these tiny hills and grooves  entangle or interlock with each other opposite motion. When we attempt to move any one object, we have to apply force to overcome interlocking.

* Friction increases as the mass increases.

* Friction produce heat.

The factor of friction depends on the :-
• Nature of surfaces in contact.
• Mass of objects in contact.

TYPES OF FRICTION 

(i) Static friction :- Static friction is the frictional force that exists between two bodies as long as they are relatively at rest,  even though an external force is acting upon them.

FLUID FRICTION
• The frictional force exerted by fluids is called drag.
• This drag depends upon the shape, material and the speed of the object.
• Fluids friction is minimum for objects that are narrow in front and broad at the back. This is called a streamlined shape.
Example - Boats and Ships have streamlined bodies to help them move through water easily.
* The substances which reduce friction is call lubricants.

ADVANTAGES OF FRICTION
Friction plays an important role in our daily life. Some examples to prove this are given below : -
• Friction between our feet or foot wear and the ground helps us to work without sleeping. That is why it is difficult to walk on Ice or on a smooth polished floor.
• The friction between the treaded tires and the rough surface of the road enables vehicles to move safely on the road. That is why we observe more incidents of vehicles skidding on greasy or wet roads on rainy days.
• Friction between the brake shoe and the wheel slows down or stops a  moving vehicle.
• We are able to light a matchstick, sew clothes, tie a knot or fix a nail on the wall because of friction.
• Friction between a pencil or a pen and paper helps us to write. That is why we find it difficult to write on smooth surfaces like plastic, glass, etc. It is friction that allows your teacher to write on the blackboard.

DISADVANTAGES OF FICTION
The advantages of friction are numerous. But it has some disadvantages too:-
• Friction wears out the rubbing surfaces like screws and ball bearing of machines or soles of shoes.
• The objects in contact require more force to overcome friction between their rubbing surfaces. This results in wastage of energy.
• You already knowo that friction generates heat. This heat raises the temperature of a machine, sometimes causing damage to its parts. Hence, proper arrangements have to be made to keep the machine cool. For example, insulation of windings of machines like mixer grinders, hair dryers, etc. may get spoiled due to friction and the motor may burn.

INCREASING FRICTION
We use different techniques to increse friction : -
• Roughen the surfaces
• Dry the surfaces
• Use brake pads

REDUCING FRICTION