TEXTBOOK EXERCISES
I. Choose the best
answer.
1. If we apply force against the
direction of motion of the body, then the body will
a.
stop moving
b.
move with an increased speed
c.
move with a decreased speed
d.
move in a different direction
[Answer: (a) stop moving]
2. Pressure exerted by a liquid
is increased by
a.
the density of the liquid
b
the height of the liquid column
c.
Both a and b
d.
None of the above
[Answer: (c) Both (a) & (b)]
3. Unit of pressure is
a.
Pascal
b.
Nm–2
c. Poise
d.
Both a and b
[Answer: (d) Both (a) & (b)]
4. The value of the atmospheric
pressure at sea level is
a.
76 cm of mercury column
b.
760 cm of mercury column
c.
176 cm of mercury column
d.
7.6 cm of mercury column
[Answer: (a) 76 cm of mercury column]
5. Pascal’s law is used in
a. hydraulic lift
b. brake system
c. pressing heavy bundles
d.
All the above
[Answer: (d) All the above]
6. Which of the following liquids
has more viscosity?
a.
Grease
b.
Water
c.
Coconut oil
d.
Ghee
[Answer: (a) Grease]
7. The unit of viscosity is
a.
Nm2
b.
poise
c.
kgms–1
d.
No unit
[Answer: (b) poise]
II. Fill in the
blanks.
1.
The pressure of a liquid column increases with the depth of the column.
2.
Hydraulic lift works under the principle of Pascal’s Law.
3.
The property of surface tension of a liquid surface enables the water droplets to move
upward in plants.
4.
A simple barometer was first constructed by Torricelli.
III. State true or
false. If false, correct the statement.
1. Force acting on a given area is called pressure.
2.
A moving body comes to rest due to friction alone.
3.
A body will sink if the weight of the body is greater than the buoyant force.
4.
One atmosphere is equivalent to 1,00,000 newton force acting on one square
metre.
5.
Rolling friction is slightly greater than the sliding friction.
6.
Friction is the only reason for the loss of energy.
Correct statement: Rolling friction is slightly
lesser than the sliding friction.
7.
Liquid pressure decreases with the decrease of depth.
8.
Viscosity depends on the pressure of a liquid.
IV. Match the
following.
a.
Static friction - Viscosity
Kinetic friction - Least friction
Rolling friction - Objects are in motion
Friction between the liquid layers - Objects
are sliding
Sliding
friction - Objects are at rest
[Answer:
i-e, ii-c, iii-b, iv-a, v-d]
i. Static
friction (e) objects are at rest
ii.
Kinetic friction (c) objects are in motion
iii.
Rolling friction (b) least friction
iv.
Friction between the liquid layers (a) viscosity
V. Sliding
friction (d) objects are sliding
b.
Barometer - reduce friction
Increasing area of contact - Atmospheric pressure
Decreasing area of contact - cause of friction
Lubricants - increases friction
Irregular surface - decreases friction
[Ans : i
- b, ii - d, iii - e, iv - a, v - c]
i.
Barometer (b) atmospheric pressure
ii.
Increase friction (d) increasing area of contact
iii.
Decrease friction (e) decreasing area of contact
iv.
Lubricants (a) reduce friction
v.
Irregular surface (c) cause of friction
V. Complete the
analogy.
1.
Knot in a thread : Static friction :: Ball bearing : rolling friction
2.
Downward force : Weight :: Upward force offered by liquid :
VI. Numerical
Problem.
1. A stone weighs 500 N.
Calculate the pressure exerted by it, if it makes contact with a surface of
area 25 cm2.
Answer:
Given : Weight of a stone F = 500 N
Area A=25 cm2 = 25 × 10-4 m2
To find : Pressure P =?
Formula :
Pressure P = F/A
= 500 / [25×10-4]
Solution:
Pressure P = 20 × 104 Nm-2 (or) 20 × 104
Pa
2. In a hydraulic lift,
the surface area of the input piston is 10 cm2. The surface area of the output
piston is 3000 cm2. A 100 N force applied to the input piston raises the output
piston. Calculate the force required to raise the output piston.
Answer:
Solution
:
Pressure input on piston,
P = F/A
= 100/ [10×10-4] = 105 N
According to Pascal’s law
P= F/A
105 = F /
[3000×10-4 ] = [ F×104 ] / 3000
104 × F = 105 × 3000
F = 3000 × 101
F = 3 ×
104N
VII. Consider the
statements given below and choose the correct option.
a. Both assertion and reason are true and reason is the correct explanation of assertion.
b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
c. Assertion is true, but reason is false.
d. Both assertion and reason are false.
1.
Assertion: Sharp knives are used to
cut the vegetables.
Reason: Sharp
edges exert more pressure.
[Answer: (a) Both assertion and reason are true and the reason is
the correct explanation of the assertion]
2.
Assertion: Broad straps are used in
bags.
Reason: Broad
straps last for long.
[Answer: (b) Both assertion and reason are true, but the reason is
not the correct explanation of the assertion]
Correct explanation: The weight of the bags falls on larger area of
shoulder. So lesser pressure is produced.
3.
Assertion: Water strider slides
easily on the surface of water.
Reason: Water
strider experiences less buoyant force.
[Answer: (b) If both
assertion and reason are true, but reason is not the correct explanation of
assertion]
Correct
explanation: It is due to the surface
tension of water.
VIII. Answer very
briefly.
1. Give two examples to verify
that a force changes the shape of a body.
Answer:
Force can change the static condition of a body.
(i) If you squeeze a sponge, its shape changes.
(ii) If you pull a rubber band, it becomes longer.
2. Give two examples to verify
that a force tends to change the static condition of a body.
Answer: Force can change the static condition of a body.
(i) A rest rubber ball begins to move, when a force applied on
it.
(ii) Player applies a force on the stationary football while
taking a penalty kick in football match. The force applied by player makes the
football move towards the goal.
3. How do you feel when you touch
a nail immediately after it is hammered into a wooden plank? Why?
Answer: The nail becomes hot due to friction. Friction changes kinetic energy
to heat.
4. How does the friction arise
between the surfaces of two bodies in relative motion?
Answer: The force of friction is arised by the interlocking of the
irregularities of the two surfaces.
5. Name two instruments which
help to measure the pressure of a fluid.
Answer:
(i) Manometer
(ii) Pressure gauge.
6. Define one atmosphere.
Answer: The pressure exerted by the mercury column is considered as the
pressure of magnitude ‘one atmosphere’ (1 atm).
7. Why are heavy bags provided
with broad straps?
Answer: Broader straps are provided on a back-pack for giving less pressure on
the shoulders by providing a larger area of contact with the shoulder.
8. How does surface tension help
a plant?
Answer: Water molecules rise up due to surface tension. Xylem tissues
are very narrow vessels present in plants. Water molecules are absorbed by the
roots and these vessels help the water to rise upward due to “capillarity
action” which is caused by the surface tension of water.
9. Which has greater viscosity,
oil or honey? Why?
Answer: Honey has greater viscosity.
Reason: Thicker liquids are more viscous than thinner liquids. As
honey has greater viscosity, more frictional force will be acting on it.
IX. Answer briefly.
1. Define friction. Give two
examples of the utility of friction in day to day life.
Answer:
Friction
: Friction is a force that slows down moving objects or prevents
stationary objects from moving.
Examples
of the utility of friction in day to day life.
(i) Cars and buses are able to move safely on the road because
of friction between the treaded tyres and the surface of the road.
(ii) We are able to write on paper only with the help of
friction between the pencil or pen and paper.
2. Mention any three ways of
minimising friction.
Answer:
(i) By
using lubricants : These are applied to
surfaces to reduce the friction between the surfaces.
Ex : Oil, wax, grease and castor oil.
(ii) With
the help of polishing the surface :
We sprinkle fine powder on the carrom board and then we polish
its surface to make smooth so that the striker slides easily on the surface.
(iii) By
using ball bearing :
We use leadshots in bearing of a cycle hub because rolling
friction is smaller than sliding friction.
3. State Pascal’s law and mention
its applications.
Answer:
(i) In an automobile service station, the vehicles are lifted
upward using the hydraulic lift, which works as per Pascal’s law.
(ii) The automobile brake system works according to Pascal’s
law.
(iii) The hydraulic press is used to make the compressed bundles
of cotton or cloth so as to occupy less space.
4. Why is a ball bearing used in
a cycle hub?
Answer: The rolling friction is smaller than sliding friction, sliding
is replaced by rolling with the usage of ball bearings. So lead shots are used
in the bearing of a cycle hub to reduce the friction.
X. Answer in
detail.
1. Friction is a necessary evil -
Explain.
Answer:
Friction is a necessity in most of our day to day activities. It
is desirable in most situations of our daily life.
(i) We can hold any object in our hand due to friction.
(ii) We can walk on the road because of friction. The footwear
and the ground help us to .walk without slipping.
(iii) Writing easily with a pen on paper is due to friction.
(iv) Automobiles can move safely due to friction between the
tyres and the road. Brakes can be applied due to frictional resistance on brake
shoes.
(v) We are able to light a matchstick, sew clothes, tie a knot
or fix a nail in the wall because of friction.
Though it is giving a negative effect, in most of our day to day
life friction helps us to make our life easy. So, it is called as “necessary
evil”.
Disadvantages
of friction :
(i) Friction wears out the surfaces rubbing with each other,
like screws and gears in machines or soles of shoes.
(ii) To overcome the friction an excess amount of effort has to
be given to operate a machine. This leads to wastage of energy.
2. Give the different types of
friction and explain each with an example.
Answer:
Friction can be classified into two basic types:
(i) Static friction
(ii) Kinetic friction.
(i) Static
friction: The friction experienced by the
bodies, which are at rest is called static friction. (E.g : All the objects
rigidly placed to be at rest on the Earth, a knot in a thread.)
(ii) Kinetic
friction: Friction existing during the motion
of bodies is called kinetic friction.
Further, kinetic friction can be classified into two:
(i) Sliding friction
(ii) Rolling friction.
(i) Sliding
friction: When a body slides over the surface
of another body, the friction acting between the surfaces in contact is called
sliding friction.
(ii) Rolling
friction: When a body rolls over another
surface, the friction acting between the surfaces in contact is called rolling
friction.
Rolling friction is less than sliding friction. That is why
wheels are provided in vehicles, trolleys, suitcases etc.
3. Describe an experiment to
prove that friction depends on the nature of a surface.
Answer:
To understand about the frictional force between the layers of
liquid in motion.
Materials
required: Different kinds of liquid (coconut
oil, honey, water, ghee), glass plates - 4 nos.
Procedure
:
(i) Take a small quantity of different kinds of liquid like
coconut oil, honey, water and ghee etc., in a cup.
(ii) Place one drop of each liquid on a separate glass plate.
(iii) Next, gently raise one end of the glass plate, one by one,
so as to allow the liquid to slide down the smooth surface of the plate.
(iv) Observe the speed of each liquid.
Observation: Each liquid moves with a different speed. Water flows faster
than other liquids. Coconut oil flows with a moderate speed. Ghee flows very
slowly.
Inference:
Between the layers of each liquid, in motion, there is a frictional force
parallel to the layers of the liquid. This frictional force opposes the motion
of the liquid layers while they are in motion.
4. Explain how friction can be
minimised.
Answer:
(i) Using
lubricants :
(1) A substance, which reduces the frictional force, is called a
lubricant.
E.g : Grease, coconut oil, graphite, castor oil, etc.
(2) The lubricants fill up the gaps in the irregular surfaces
between the bodies in contact. This provides a smooth layer thus preventing a
direct contact between their rough surfaces.
(ii) Using
ball bearing :
Since, the rolling friction is smaller than sliding friction,
sliding is replaced by rolling with the usage of ball bearings. We can see lead
shots in the bearing of a cycle hub.
5. Describe an experiment to
prove that the pressure in a liquid increases with depth.
Answer: Take a plastic bottle. Punch three holes on its side in the
same direction, but at different heights. Now pour some water into it and let
it flow through the holes. Observe the flow of water.
Inference
: The water comes out from all the holes with a different force
and falls on the table at points that are at variable distances from the
bottle. Water from the lowest hole comes out with the greatest force and falls
at a point that is at the maximum distance from the bottle. Water from the
topmost hole comes out with the least force and falls at the point that is at
the minimum distance from the bottle.
Reason: This activity confirms that the pressure in a liquid varies
with the depth of the point of observation in it.
XI. Higher Order
Thinking Questions.
1. Why is it not advisable to use a fountain pen
while travelling in an aeroplane?
Answer: Fountain pens are built in such a way that the pressure inside
them balances the atmospheric pressure at sea level. Since atmospheric pressure
decreases with an increase in height above sea level, the pressure inside the
pen turns out to be much greater than the air pressure in an aeroplane and the
pen stalls leaking.
2. Is there any possibility of
making a special device to measure the magnitude of friction directly?
Answer: Yes. Tribometer is a special device to measure the magnitude of
friction directly.
3. Vidhya feels that mercury is
costly. So, instead of mercury she wants to use water as a barometric liquid. Explain
the difficulty of constructing a water barometer.
Answer:
(i) Mercury is commonly used in barometers because of its high
density means the height of the column can be a reasonable size to measure atmospheric
pressure.
(ii) A barometer using water, for instance, would need to be
13.6 times taller than a mercury barometer to obtain the same pressure
difference.
(iii) This is because mercury is 13.6 times more dense
than water.
XII. Project Work.
Observe the devices, gadgets or
things around you. List out the types of friction involved in each device. How
would you minimise the friction? Record your observations and discuss your
results with your classmates.
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