FRICTION
Looking Back
Say whether the statements are true or false.
1. False 2. True 3. True 4. False 5. d
Exercises:
A. 1. d 2. c 3. a 4. d 5. a 6. a 7. a 8. a
B. 1. less 2. rolling 3. drag 4. more 5. increase 6. decrease 7. rolling
8. streamlined
C. 1. True 2. False 3. False 4. True 5. True 6. True 7. True
D. 1. Friction is a contact force that tends to stop a moving object and also tends to stop a stationary
object from moving.
2. The force of friction acts in a direction opposite to the direction of motion of an object.
3. The friction that exists between a stationary object and the surface on which it is resting is called
static friction. It prevents an object from sliding. Sliding friction occurs when one surface slides over
another. Sliding friction is less than static friction.
4. When a wheel moves on a surface, rolling friction comes into play.
5. Drag is the friction due to fluids. The frictional force which opposes the motion of an object moving
through fluids such as air and water is called drag.
6. Without friction, it will be impossible to walk. We would slip and fall every time we attempt to
walk or run. It is easy to slip on a wet polished floor because there is very little friction on such a floor.
7. The lubricant fills the grooves and crevices (surfaces of contact) in the moving surfaces. Instead of
friction between solid surfaces, fluid friction comes into play. Fluid friction is much less than solid
friction. Thus lubrication reduces friction.
E. 1. Factors that affect friction are:
(i) The nature of the surfaces that are in contact: The irregularities on a surface are the cause for
friction when one surface moves over another. A rough surface has more irregularities and
hence friction is usually greater between rough surfaces than it is between smooth surfaces.
(ii) Force of attraction between some atoms on the two surfaces, which has to be overcome.
(iii) Force exerted by an object: Greater the force the greater the friction. Friction increases with
increase in mass.
2. To show the difference in friction on different surfaces, we will measure the force required to move
a block of wood across different surfaces using a spring balance. The set up is as shown in figure
given in the course book. The procedure is:
(i) Attach a hook to a block of wood.
(ii) Tie one end of the string to the hook and the other end to the spring balance.
(iii) Place the block of wood on a rough cemented floor and pull the spring balance.
(iv) Keep increasing the force till the block just starts moving on the surface. Take note of the
reading on the spring balance at that time.
(v) Next place the block on a smooth surface and pull the spring balance. Repeat step iv.
(vi) Test on other surfaces and tabulate the results.
(vii) Higher reading indicates higher force has been applied to overcome friction. Hence higher the
reading greater the frictional force between the surfaces.
3. Advantages of friction:
(i) Without friction, we would slip and fall every time we attempt to walk or run. There is very little
friction on a wet polished floor. That is why it is easy to slip on a wet floor.
(ii) It would not be easy to light a matchstick without friction between the head and the side of the
matchbox.
(iii) Cars and buses are able to run on the roads because of friction between the tyres and the
road.
4. Disadvantages of friction. Friction is a nuisance sometimes.
(i) Energy is wasted in overcoming friction.
(ii) Friction causes wear and tear of the rubbing parts. Tyres of vehicles and soles of footwear wear
out because of friction. Moving parts of machines wear out due to friction.
(iii) Friction produces heat because energy wasted in overcoming friction is converted to heat. If
friction in a machine is high, the heat produced can damage the parts. Arrangements have to be
made to cool down fast moving machines to avoid the damage. Cars have a cooling system to
keep the engine cool.
(iv) Forest fires are caused due to friction between branches of trees rubbing against each other.
5. Sometimes it is desirable to increase friction between surfaces for example to avoid slipping. This
can be done by making the surfaces rough in the following ways.
(i) By making the moving surfaces rough, for example, tyres have designs and patterns with
grooves on the surface to increase resistance with the road. This prevents the slipping of the
tyres on wet ro (ii) Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is
then easier to walk on the ground.
(iii) To increase friction, spikes are provided in the soles of the shoes used by players and athletes.
6. Friction can be minimised by making the surfaces smooth in several ways:
(i) By using lubricants such as oil or grease. This helps because fluid friction is much less than solid
friction.
(ii) By using wheels and ball bearings. The use of wheels between surfaces moving over each
other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because
of the balls, the steel surfaces can move over each other as rolling friction is less than sliding
friction.
(iii) By making surfaces smooth by polishing them. Polishing removes the grooves and crevices that
exist on the surfaces which cause friction.
Higher-order Thinking Skills
1. The friction between two surfaces will depend on the area of contact. When a body is rolling there
is only a point of contact between the rolling body and the surface, while a sliding body is in
complete contact with the surface. The rollers reduce the area of contact with the surface and
therefore reduce friction. Hence rolling friction is lesser than sliding friction.
2. The air resistance of a parachute jumper is increased to make sure that the person land gently,
slowly and safely. A parachute opens out behind the person and creates a large surface area of
material with a large amount of drag. It is so designed that the person lands at a relatively low
speed.
3. Water can corrode machine parts made of metals like iron in the presence of atmospheric oxygen.
This could damage the machine components. Further water can evaporate at 100°C, hence it would
evaporate due to the heat of the machinery, while oil and grease have higher temperatures of evaporation. Hence they are preferred.
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