Page 118 - Science Class 08

The brakes of vehicles cannot work without oil, grease or graphite is poured between
fric on. The fric on between brake-shoe and
brake drum slows down the vehicle when
brakes are applied.
Fric on helps us wri ng on a paper or on the
black board.
Fric on is necessary to insert a nail in the wall.
Fric on helps us making a knot.
Disadvantages of Fric on : Fric on prevents
machinery from moving freely. Therefore, extra
energy is used to overcome the force of fric on.
The fric on caused between the surface of two
machinery parts produces heat. This raises the
temperature of the machine which may damage
the machine.
For example, running a motor con nuously for
many hours may burn the motor. It is,
therefore, a proper cooling arrangement is
made to keep the engine of cars or buses, so as
to save it from the damage caused by excess
hea ng.
The soles of your shoes wear out in a few
months due to fric on.
The moving parts of a machine wear out with
me due to fric on.
Some Way of Minimising Fric on : You have
studied that fric on causes wastage of energy,
fric on wears out moving parts of a machine
and it can also heat up the moving parts of a
machine which can damage the machine.
Therefore, to minimise the damage and to
increase the efficiency of the machine, we
minimise the fric on.
Let us study about the ways we can minimise
fric on.
v Polishing : Fric on can be minimised by
polishing the rough surface of the moving,
object.
v Lubrica ng : It is the most commonly used
method to reduce fric on. In this method,
118
the surfaces of moving parts. Oil or grease
is used in simple machines, motor vehicles,
fans, sewing machines, tractors, etc. to
reduce fric on. Graphite is used in the
machines where the moving parts reach
very high temperature.
v Using ball bearing and wheels : The
magnitude of fric on also depends on the
area of the surface covered by two
surfaces. The more the area is in contact
the more will be fric on. A wheel has a
li le contact area with the ground or any
other surface, therefore, there is less
fric on. That is why vehicles are equipped
with wheels. Nowadays, people today
prefer to have briefcase equipped with
roller wheels.
This principal is also made use of in ball
bearings which contain steel balls or rollers.
Ball bearing are used between the wheel
and axle of a car reduce fric on to a greater
extent.
v Stream linings : A fish has a streamlined
body that helps it moving with less effort.
This principal is also used in making
aeroplanes, ships, boats, missiles, rockets,
etc. The streamlined shape of aeroplanes
reduce the fric on between air and
aeroplanes. Similarly, streamlined body of a
ship also reduce the fric on between the
ship and water. The type of shape also helps
a ship in making passage in water easily.
Way of Increasing Fric on : You have also
studied about the advantages of fric on.
Therefore, in some cases we need to increase
fric on to get more advantages.
Let us see the ways we can increase fric on:
v Sportsman wear spikes to run on ground
with ease. The soles of spikes have nails
Science-8