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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