FRICTIONS

 1. INTRODUCTION :: 

(i) When two surfaces in contact have relative motion or have tendency of motion with each other then a force acts at the point of contact of the object and this force is called frictional force. 

(ii) The force of friction is always in a direction opposite to which the body tends to move 

(iii) It is paralel to the surface 

(iv) Frictional force is independent of the area of surface in contact 

(v) The force of friction depends on the nature of material of the surface in contact. 

(vi) Friction is a non-conservative force i.e. work done against friction is path dependent. 

(vii) Generally many of us have a misconception that friction opposes the motion of a moving body but it favours the motion of a body. It opposes the relative motion between the two bodies. When a person walks forward, he pushes the ground backward. The rough surface of ground exerts a forward force which causes the motion of the person.

2. TYPES OF FRICTION 

Friction is of two types 

(a) Static friction (b) Kinetic friction 

Now let us have some discussion on these types of friction 

2.1 Static friction : 

(i) Frictional force between two surfaces when there is no relative motion between them is called static friction. 

(ii) Static friction is self adjusting in nature. It adjusts its magnitude in such a way that together with other forces applied on a body, it maintains relative rest between two surfaces. 

(iii) The value of static friction lies between 0 and μₛN i.e 0 ≤ fₛ ≤ μₛN where μₛ is coefficient of static friction and N is normal force 

(iv) There is no energy loss due to static friction. We may perform work against static friction or static friction may perform work. 

2.2 Kinetic friction : 

(i) Frictional force between the two surfaces which are in relative motion is called kinetic friction. 

(ii) The magnitude of the kinetic friction is proportional to the normal force acting between the two surfaces i.e. fₖ ∝ N   fₖ = μₖN  where μₖ is coefficient of kinetic friction. 

(iii) This force of kinetic friction always. acts opposite to the direction of motion. 

(v) Work done against kinetic friction is nonconserved i.e. converted into heat. So in case of kinetic friction, there is always an energy loss

NOTE : If nothing is specified for μₖ and μₛ then we assume that both are equal. Theoretically

μₖ < μₛ