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If we slide or try to slide a body over a surface the motion is resisted by a bonding between the body and the surface. This resistance is represented by a single force and is called friction. The force of friction is parallel to the surface and opposite to the direction of intended motion.

Types of Friction

(1) Static friction: The opposing force that comes into play when objects are at rest.
       (i) In this case static friction F = P.
      (ii) Static friction is a self-adjusting force because it changes itself in accordance with the applied force.


(2) Limiting friction: The maximum value of static friction upto which body does not move is called limiting friction.

(i) The magnitude of limiting friction between any two bodies in contact is directly proportional to the normal reaction between them.

(ii) Direction of the force of limiting friction is always opposite to the direction in which one body is at the verge of moving

(iii) Coefficient of static friction:
      (a) μs is called coefficient of static friction.
      (b) Dimension: Dimesionless
      (c) Unit: It has no unit.
      (d) Value of μs lies in between 0 and 1
      (e) Value of μ depends on material and nature of surfaces in contact.
      (f) Value of μ does not depend upon apparent area of contact.

(3) Kinetic or dynamic friction: If the applied force sets the body in motion, the friction opposing the motion is called kinetic friction.

(i) Kinetic friction depends upon the normal reaction.
              43  , where μk is called the coefficient of kinetic friction
(ii) Kinetic friction is always lesser than limiting friction 44
Thus we require more force to start a motion than to maintain it against friction. This is because  when motion has actually started, irregularities of one surface have little time to get locked again into the irregularities of the other surface.

(iii) Types of kinetic friction: (a) Sliding friction (b) Rolling friction

  • Rolling friction is directly proportional to the normal reaction (R) and inversely

    proportional to the radius (r) of the rolling cylinder or wheel                                                45.PNG

     46.PNG is called coefficient of rolling friction. It would have the dimensions of length and would be measured in metre.

  • Rolling friction is often quite small as compared to the sliding friction.

  • In rolling the surfaces at contact do not rub each other.

  • The velocity of point of contact with respect to the surface remains zero all the times.

Graph Between Applied Force and Force of Friction



Angle of Friction.

Angle of friction may be defined as the angle which the resultant of limiting friction and normal reaction makes with the normal reaction.



Angle of Repose

Angle of repose is defined as the angle of the inclined plane with horizontal such that a body placed on it is just begins to slide.

         If α is called the angle of repose.

         α=θ i.e. angle of repose = angle of friction.


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