# FRICTION

The opposing force which acts in the opposite direction of the movement of the block is called friction .
It is of the following two type.
1. Static friction
2. Dynamic friction

1. Static friction -  It is the friction experienced by a body when it is rest. Or in other words, it is the friction when the body tends to move.

2. Dynamic friction - It is the friction experienced by a body when it is in motion . It is also called kinetic friction . The dynamic friction is of the following two types.
1. Sliding friction - It is the friction, experienced by a body when it slides over another body.
2. Rolling friction - It is the friction, experienced by a body when it rolls over another body.

3. Limiting friction -  It has been observed that when a body, lying over another body, is gently pushed, if does not move because of the frictional force. Which prevents the motion .It shows that the force of the hand is being exactly balanced by the force of friction , acting in the opposite direction. If we again push the body, a little harder, it is still found to be in equilibrium. It shows that the force of friction has increased itself so as to become equal and opposite to the applied force. Thus the force of friction has a remarkable property of adjusting its magnitude, so as to become exactly equal and opposite to the applied force , which tends to produce motion.
There is, however, a limit beyond which the force of the friction cannot increase . if the applied force exceeds this limit, the force of friction cannot balance it and the body beings to move, in the direction of the applied force. The maximum value of frictional force, which comes into play, when a body just begins to slide over the surface of the other body , is known as limiting friction . it may be noted that when the applied force is less than the limiting friction, the body remains at rest, and the friction is called static friction, which may have any value between zero and limiting friction.

4. Normal friction - It has been experienced that whatever a body, lying on a horizontal or an inclined surface, is in equilibrium, its weight acts vertically downwards through its center of gravity. The surface, in turn, exerts an upwards reaction on the body. This reaction which is taken to act perpendicular to the plane, is called normal reaction and is generally, denoted by R. It will be interesting to know that the term 'normal reaction'  is very important in the field of friction, as the force of friction is directly proportional to it.

5.  Angle of friction -  Consider of body of weight W resting on an inclined plane. We know that the body is in equilibrium under the action of the following forces :
1. Weight (W) of the body, acting vertically downwards,
2. Friction force (F) acting upwards along the plane, and
3. Normal reaction (R) acting at right angles to the plane.

6. Coefficient of friction - It is the ratio of limiting to the normal reaction, between the two bodies, and is generally denoted by µ.

Mathematically, coefficient of friction,
F
µ   =    ------         or   F=µR
R
where                     θ = Angle of friction
F= Limiting friction, and
R=  Normal reaction between the two bodies,

7. Laws of friction -
1. Laws of static friction -
(a) The force of friction always acts in a direction, opposite to that in which the body tends to move, if the force of friction would have been absent.
(b) The magnitude of the force of friction is exactly to the forces, which tends to move the body.

2. Laws of kinetic of dynamic friction -
(a) The force of friction always acts in a direction, opposite to that in which the body is moving.
(b) The magnitude of kinetic friction bears a constant ratio to the normal reaction between the two surfaces. But this ratio is slightly less than in case of limiting friction.