L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
PHYSICS - GRADE: VI
FRICTION
FRICTION is a contact force that opposes the relative motion between the two surfaces
in contact with each other.. It always acts opposite to the direction of motion.
OPPOSES MOTION
PRODUCES HEAT
PROPERTIES OF
FRICTIONAL FORCE
CAUSES WEAR AND TEAR
INDUCED BY SOLIDS, LIQUIDS
AND GASEOUS STATE
FACTORS AFFECTING FRICTIONAL FORCE:
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Depends upon the smoothness of the surface.
Solids, liquids and gases all exert frictional force.
Does not depend upon shape or area in contact .
Depends on normal reaction of weight of the object.
Normal reaction force.
Normal force is the force exerted on an object that is in
contact with another stable object. The stable object supports the object that
is in contact with it.
For eg.: If a book is kept on the table, the table exerts an upward force upon
the book. This force exerted by the table supports the weight of the book such
that it rests in its place.
TYPES OF FRICTION
Static friction
Rolling friction
Sliding friction
It is observed when
a heavy glass slab
placed on a table,
just begins to move
It is seen in a
rolling ball
A heavy glass
slab moving
on a smooth
table top.
Static Friction > Sliding Friction > Rolling Friction
It is easier to roll an object than to slide an object. So sliding is replaced by rolling with
the use of ball bearing. For eg. : Friction in machines can be decreased by the use of
ball bearings in between the moving surfaces. Also it is convenient to pull luggage fitted
with rollers.
( Here the black arrow represents the motion)
ADVANTAGES OF FRICTION
 Friction prevents us from slipping on the ground while walking. There is force of
friction acting between the soles of our shoes and the ground.
 Smooth running of vehicles on the road is due to the frictional force acting between
the tyres and the road.
 We can lift and hold things due to the force of friction between the object and our
hands.
 We are able to write with pencil on paper due to the force of friction acting between
the pencil/pen and paper. The same holds good when we write with chalk on the
blackboard.
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When the brakes are applied, friction between the brake pad and the brake disc
helps to reduce the speed of the vehicle.
Some other examples of advantages of friction are burning of a matchstick, climbing
a tree without slipping, etc.
WAYS TO INCREASE FRICTION
 Soles of the shoes of players are provided with grooves and spikes to increase
friction.
 Tyres of the vehicles are grooved so that they don’t skid.
 Handles of bats and racquets are made of rough materials to ensure strong grip.
 Playgrounds of children are made rough with lots of grass so that they do not slip
while running or playing.
 Grinding stones of flour mills are made rough to increase friction.
 Sand is spread over slippery surface of roads in rainy season.
 Wrestlers and kabaddi players rub their hands with soil for better and firm grip.
DISADVANTAGES OF FRICTION
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Wears away the soles of the shoes.
Wears out tyres of vehicles.
As it opposes motion, velocity of a body moving on a surface decreases and stops.
For eg.: Friction between the water and the moving boat brings the boat to a stop
when we stop rowing it.
Parts of machines get damaged due to heat that is produced due to friction.
Friction causes wastage of energy as a lot of fuel is wasted to overcome the
frictional force between the road and the wheels of the car. Also the machines have
to do a lot of mechanical work to overcome friction which results in wastage of fuel.
Sometimes friction produces undesirable noise.
METHODS OF REDUCING FRICTION
 Streamlining the shape of the object.
For eg.: In an aircraft, the structure of the aerofoil is streamlined to reduce the
friction caused due to air drag.
 By providing wheels/ball bearings to roll over a surface instead of sliding because
rolling friction is always less than the sliding friction.
For eg.: Trolleys used in shopping malls.
 Some materials like oil and grease when applied between the two surfaces
minimizes the effect of frictional force. Such materials which are used are called as
lubricants.
 Friction can also be reduced by polishing the surfaces in contact.
 By sprinkling a soft, slippery fine powder on the surfaces.
For eg.: a small quantity of boric powder is applied on the carrom boards to reduce
the friction between the board and the coins.