FRICTION

FRICTION :-

                   When on a solid surface, another solid is rubbed a force is created between the two solids which acts in the opposite direction of motion or tries to obstruct the motion of the object, this force is called frictional force. This phenomenon is called Friction.

Friction plays an important role in our daily life. It would not be possible to walk without friction between our foot and floor. Vehicles are able to run on roads because of the friction between the wheels and road. 

TYPES OF FRICTION:-

1 Static friction
2 Dynamic friction 

1. STATIC FRICTION:-

The friction between two solid objects when at rest is called static friction.

Eg. Static friction can prevent an object from sliding down on a sloped surface.

LIMITING FRICTION :-

When the frictional force (F) is equal to the applied pulling force (P) then the friction between two surfaces is known as limiting friction. (i.e F=P)

2. DYNAMIC FRICTION :-

It is the friction between two objects, when are in motion is called dynamic friction. It is also called kinetic friction. 

 SLIDING FRICTION:-

It is the friction experienced by an object when its slides over  another object. Sliding friction is always less than limiting friction.

ROLLING FRICTION :-

It is the friction that occurs when a circular object such as a ball or roller rolls on a flat surface. Rolling friction is less than sliding friction. (ball or roller bearing)

 Forces acting on a body when a pulling force is applied to move
• Weight of the block acting vertically downward (W)
• The normal reaction which acts upwards (R)
• The applied pulling force (F)
• The frictional force (Ff)

When the body is about to move W=R, F=P
When pulling force is increased the body starts to move.

LAWS OF FRICTION :-

• Frictional force is directly proportional to the normal reaction    between contacting surfaces.

• Frictional force acts opposite to the direction of motion.
• Frictional force depends on the nature of contacting
surfaces.
• Frictional force is independent over the area and shape of contacting surfaces.

COEFFICIENT  OF FRICTION :-

It is a ratio between the frictional force to the normal reaction when the body is just about to move but at equilibrium. It is represented by symbol  ߎ(read as ‘meu’).

Therefore

Co -efficient of friction =.   Limiting friction or force

 Normal friction or force

ߎ(meu) = F/W or Ff/R

                      

ANGLE OF FRICTION :-

The forces acting on a body when it is just about to move by the application of a pulling force are W, R, P and F. The forces 'R' and 'F' are compounded and we get the resultant force 'S'. The angle formed by 'S' with 'R' is the angle of friction.

Therefore

tan ϴ = F/W         tan ϴ = ߎ
           

ANGLE OF REPOSE :-

A body placed on an inclined surface remains at rest till the angle of inclination equals the angle of friction. When it exceeds the body starts sliding down. This is known as angle of repose.

MOTION UP THE PLANE:-

When 𝛳 > Ⲫ a force must be applied to keep the body in equilibrium. The applied force may be parallel to the plane, horizontal or at an angle to the plane itself.

When the body is at the point of motion up the plane the frictional force ‘F’ acts down the plane.

Forces acting are W,R,P and F. The weight force ‘W’ is resolved into two components of W cos 𝛳 perpendicular to the plane acting downwards and W sin 𝛳 acting parallel to the plane downwards.

                          F/R  = μ = tan φ
                                            

Where Ⲫ is the angle of friction.

∵  P = F + W sin 𝛳and       R = W cos 𝛳 

∵  P = uR + W sin 𝛳 = u(W cos 𝛳) + W sin 𝛳

∵  P = W x tan Ⲫ  cos 𝛳 + W cos 𝛳

            = W x sin Ⲫ  x cos + W cos θ
                       cos Ⲫ
 = W x sin Ⲫ + cos +W cos Ⲫ x cos 𝛳
                                 cos Ⲫ
           = W [Sin(𝛳+ Ⲫ)]
                    Cos Ⲫ

Similarly when the body is about to slide down the plane the applied force P must be equal to
             = W [Sin(𝛳+ Ⲫ)] 
                        Cos Ⲫ

To keep the body under equilibrium when the body is about to move up the plane by the action of an applied force the applied force P =  W [Sin(𝛳+ Ⲫ)]  
                       Cos Ⲫ

 and the force necessary to be applied to the body to prevent it from sliding down the plane will be 
           P = W [Sin(𝛳+ Ⲫ)] 
                          Cos Ⲫ 
                   
                                                      
Note : Under all circumstances

 = W x [sin(𝛳+ Ⲫ) < P < =       W [Sin(𝛳+ Ⲫ)]  
                 Cos Ⲫ                                  Cos Ⲫ 

ADVANTAGES OF FRICTION :-

1 Helps us to walk without slipping.

2 Used to stop vehicles when brakes are applied.

3 Movement of vehicles due to friction between revolving

wheels with tyres and the road.

4 Power transmission using gear drive or belt pulley drive.

5 Using friction we can sharp any object and also to hold

it.

6 Nails and screws are held in wood by friction.

7 Heat is produced when two rough surfaces are rubbed

against each other.

DISADVANTAGES OF FRICTION :-

1 It causes wear and tear of the machine parts.

2 It produces heat and may cause melting of machine

parts. To avoid production of heat using of coolant is

necessary.

3 It reduces efficiency of a machine.

4 It reduces speed of the moving object. eg. spindle, shaft,

piston etc.

FRICTION CAN BE REDUCED:-

1 By using suitable lubricants (oil, grease) between the

moving parts.

2 By polishing the surface to make them smooth.

3 By using ball bearings and roller bearings.

4 By the use of wheel.