If the switch off the engine of car it will stop after moving a certain distance. It means that some retarding force is acting on the car which stops it. The force opposing the motion of the car is called "force of friction".
(i) The frictional force is tangential to the surface in contact and always in a direction opposite to the direction of motion of the object.
(ii) Frictional force is a force opposing the relative motion between two surfaces which are in contact with each other.
(iii) The force of friction arises due to the interlocking of irregular projections on the two surfaces and is called as "force of contact".
TYPES OF FRICTION
(A) Static friction :
The frictional force that exists between the body and the surface so long as they are relatively at rest even when the external force acts, is called the static friction.
(B) Limiting friction :
(i) When there is no external force acting on the body, the frictional force is zero.
(ii) If we increase the magnitude of the external force, the static friction also increases.
(iii) At one pt, when there is a slight increase in the magnitude of the external force, the body just starts sliding over the surface.
(iv) The force of friction at the limiting stage is called the 'limiting friction.'
(v) The limiting friction depends upon the nature of the two surfaces in contact.
(C) Kinetic or Sliding friction :
(i) When the external force exceeds the limiting friction, the body just starts sliding.
(ii) The minimum force required to maintain the motion of the body over the other surface is called the "kinetic friction".
(iii) The frictional force that exists between the two surfaces when there is a uniform relative motion between them is called the "dynamic friction" or "sliding friction".
(iv) Kinetic friction is smaller than the limiting friction.
(D) Rolling friction :
(i) The frictional force that exists between the two surfaces when a body rolls over the other is called the "rolling friction".
(ii) It deforms the surface over which it rolls.
(iii) Rolling body it-self gets deformed at the pt of contact over the surface.
* Variation of frictional force with applied forces :
(i) The part OA of the gr