Friction

Friction is defined as a force which acts at the surface of separation between two objects in contact and tends to oppose the motion of one over the other. It acts tangential to the surface of separation between two bodies in contact. Friction is a force of opposition.
FORMS OF FRICTION
(i)                 STATIC OR LIMITING FRICTION: This is the maximum force that must be overcome before a body can just start to move over another.
(ii)               KINETIC OR DYNAMIC FRICTION: This is the force that must be overcome so that a body can move with uniform speed over another body.
LAWS OF SOLID FRICTION
(i)                 Friction opposes the relative motion between two surfaces in contact. It acts in a direction opposite to that of the motion.
(ii)               The force of friction increases to the same extent as the force which tends to start the motion.
(i)                  Frictional force depends on the nature of the two surfaces in contact i.e the nature of the surface and the conditions of the surfaces such as rough, smooth, polished, wet, dry, etc.
(ii)               Friction is independent of the area of the surfaces in contact.

(iii)              Friction varies directly with the normal force pressing the surfaces together. That is it is proportional to the normal reaction R
Since F is roughly proportional to the Normal Reaction R between the two surfaces in contact.
F α R ……….. (i)
F = µR ……….(ii)
where “µ” is proportional constant called co efficient of friction.
From equation (ii), divide both sides by R
F/R = µR/R
µ = F/R.
ADVANTAGES OF FRICTION
(i)                 The friction between our feet and the ground help us to walk or stop walking.
(ii)               Friction enables the automobile tires to make a firm grip with the roadway, thus aiding their movement.
(iii)             It is utilized in fan belts used over wheels or pulleys in machinery.
(iv)             It enables a screw or nail to remain in place after being screwed into position.
(v)               Friction enables the brakes to stop the car, and the locomotive to pull the train.
(vi)             It is used in grid stone to sharpen knives, chisels etc.
DISADVANTAGES OF FRICTION
(i)                 It causes loss of energy in machines.
(ii)               It causes wear and tear on the moving parts of machines.
(iii)              It causes the heating of engines.
METHODS OF REDUCING FRICTION
(i)                 The use of lubricant (oil, grease, air, graphite etc.)
(ii)               The use of ball or roller bearing.
(iii)             The streamline of the body shapes of moving object. 

Example 1: A block of wood weighing 4 N is placed on a horizontal table. It is then pulled by means of a spring balance attached to one of its ends. The block just begins to move when the spring balance records a horizontal force of2.5 N.
(i)                 What is the coefficient of static friction?
(ii)               What is the frictional force when the horizontal force of 2.0 N is recorded by the spring balance?
SOLUTION
(i)                 Limiting frictional force F = 2.5 N
Normal Reaction R = 4 N
                   µ = F/R
µ =  2.5 / 4 = 0.625.
(ii)               When the applied force is 2.0 N the block will not move because 2.0 N is less than the limiting friction. The applied force is always balanced by the opposing frictional force.
 Hence frictional force = 2.0 N.

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