FRICTION Class VIII Chapter 12
INTRODUCTION -
When we go somewhere by a bicycle we often apply brakes to
slow or stop the bicycle. When we are running or walking and unknowingly under
our feet some banana peel comes, we often fall and get injured.
Look at the tyres of bicycle, bus; truck etc. like vehicles
there is special design to help in its stopping on applying brakes. The rough
surfaces help in stopping of vehicle unlike the smooth slippery surfaces. It is
due to presence of friction in between the road or surface and the tyres of the
vehicle the helps in stopping the vehicle on applying the brakes.
The force that comes in between two surfaces in contact due
to their relative motion is called friction or frictional force.
SALIENT FEATURES OF FRICTION-
Following are the salient features of frictional force-
1. The force of
friction opposes the applied force. e.g.- If you push a wooden box on a
surface, the frictional force will come into action. It opposes the push force
and reduces the net force.
2. The force of
friction results in between two surfaces in contact. The nature of surfaces (rough or smooth) or irregularities, inter-molecular force of attraction (Adhesion or Cohesion) and deformation of surface due to
weight are factors which come in action during friction.
3. Friction can
be reduced by using lubricants, polishing, using ball bearings and making the
streamlined body.
4. Friction can
be increased by making the surfaces in contact rough and increasing the weight
of moving object.
5. Friction is
necessary evil. It has many advantages as well as many disadvantages. It plays
crucial role in our day to day life.
FACTORS AFFECTING FRICTION-
The force of friction results in between two surfaces in contact. The nature of surfaces (rough or smooth), inter-molecular force of attraction (Adhesion or Cohesion) and deformation of
surface due to weight are factors which come in action during friction.
The nature of
surfaces in contact determines the amount of friction. Smooth surfaces cause less friction while rough surfaces cause
comparatively more friction. Plane Glass will produce very less friction while plane
cemented rough floor will produce
more friction. It is due to irregularities
in between two surfaces in contact.
e.g.- Drop of rainwater in glass of window remain hang on.
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| Fig. Water droplets adhered on Glass |
There comes an inter-molecular force of attraction between two surfaces in contact. The inter-molecular force between similar molecules of same material is called cohesion. The inter-molecular force
between dissimilar molecules of different materials is called adhesion.
The deformation of surface due to weight is also a factor.
When weight of moving object is increased and surface on which it is moving
becomes changed, the frictional force increases.
e.g.- It is difficult to walk on sand than on plane
surface.
FRICTION AS A
NECESSARY EVIL- There are certain advantages and disadvantages of
friction. So, it is called as a necessary evil.
Advantages-
1. Friction develops between our feet or shoes and the ground. So, it
helps in walking and running.
2. Friction helps in writing with pen, pencil on paper and on blackboard
with chalk.
3. Friction helps to light the matchstick when rubbed on the side of the
matchbox.
4. Friction that develops between tyres and the road helps the vehicles
to run on the road.
5. Friction between breaks and tyres help to stop the vehicle on applying
the force.
Disadvantages-
1. Friction slows down or stops the motion of the objects.
2. Friction generates heat. So, the running vehicles and machines become
hot. It is wastage of energy.
3. Friction causes wear and tear of the soles of shoes and slippers.
INCREASING OF FRICTION- Friction can be increased by
–
1. Making the surface more rough-
e. g.- The new tyres with more roughness give better grip than the old
weared out tyres.
e. g.- The carpets help in comfortable walking on granite floor.
e. g.- ‘Kabaddi’ players rub their hands with soil for better grip of
their opponents.
e. g. –Pens are provided with rubber grip for better holding and
comfortable writing.
e. g. – Weight lifters and Gymnasts apply powder on their hand for better
grip.
2. Increasing the weight of moving object-
e. g.- The pulling of a mat is comparatively easy than the pulling of mat
with more load kept on that.
REDUCING OF FRICTION-
1. By Polishing- due to
polishing the roughness of the surface get reduced. So, the irregularities due
to contact of two surfaces become less and friction reduces.
2. By using lubricants – A lubricant is a substance which reduces
friction. e. g.- Oil, Grease, Talcum powder and Graphite. Lubricant is applied
in machines to reduce the friction.
e.g.- The movement of doors and windows become smooth when oil is applied
on the hinges.
e.g.- The moving parts of a machine or vehicle are made smooth by using
grease.
e.g.-The coins of carom-board move faster easily on striking due to
spreading of talcum powder.
3. By using ball-bearings – The ball bearing change the sliding friction
into rolling friction. Rolling friction is less than sliding friction so it is
easy to move the vehicles, fans etc where it is found.
4. By making streamlined body design- The streamlined bodies are narrow
from front and back. It reduces the friction on moving body in air and water.
TYPES OF FRICTION-
The static friction is
exactly equal to the applied maximum force until the object does not start
moving.
B.
SLIDING FRICTION- The friction that develops when an object slides over other
is called as sliding friction.
C. ROLLING FRICTION- The friction that develops when an object rolls over the other object is called rolling friction.
D.
FLUID FRICTION (DRAG)- (Daniel Bernoulli) The liquids and gases are commonly
called as fluids. The resistance or friction that develops when a solid object travelling in liquid or gaseous medium is called fluid friction. It is also called as drag.
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| Fig. Four types of forces on flying Aeroplane |
Four types of forces on a flying aeroplane are-
1.Fluid friction /Drag
2. Thrust
3. Weight due to gravity
4. Lift or Upthrust
The viscosity (thickness) of fluid, surface
texture i.e. roughness or smoothness and shape of moving object are factors
that affect the fluid friction.
EXERCISE QUESTIONS-
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