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3. ATMOSPHERIC PRESSURE

3 ATMOSPHERIC PRESSURE

Important points to remember :

1. Force : Force is required to (1) move a stationary object (2) stop

a moving object (3) change the velocity and speed of an object

(4) change the size and shape of an object.

2. Pressure : The force applied on unit area is called pressure.

Pressure_

force

area

The unit of pressure is the newton per square metre

(N/m_).

3. Pressure affects substances in the solid state, liquid state, as well

as the gaseous state.

4. A fluid substance exerts equal pressure in all directions.

5. A fluid flows from a region of higher pressure to a region of

lower pressure.

6. The pressure exerted by the atmosphere is called atmospheric

pressure.

7. The working of a pump, a spray pump, etc., is based on the

atmospheric pressure.

Questions and Answers

Q. 1 Answer the following questions :

(1) State the effects of a force acting on an object.

Ans. The force acting on an object can (1) set the object in

motion if it is initially stationary (2) stop the object if it is initially

in motion (3) change the velocity and speed of the object (4) change

the size and shape of the object.

(2) What is pressure ? OR Define pressure.

Ans. The force applied on unit area is called pressure.

(3) State the formula for pressure. Hence, determine the

unit of pressure.

Ans. Pressure_

force

area

_ The unit of pressure_

the unit of force

the unit of area

_the newton per square metre (N/m_)

30

(4) State the factors on which pressure depends.

Ans. Pressure depends on the applied force and the area of the

surface on which the force is applied.

(5) Explain why a pointed nail can be easily hammered

into wood than a blunt one, if the same force is applied.

Ans. (1) Pressure varies inversely as the area of the surface on

which the force is applied, if the force remains constant. If the same

force is applied to surfaces having different areas, the pressure is

more on the surface having a smaller area. (2) The tip of a pointed

nail has an extremely small area, while that of a blunt nail has a

comparatively large area. Therefore, the same force will create a

large pressure on the pointed nail and it can be easily hammered

into the wood, while a very less pressure is created on the blunt nail

and it cannot be easily hammered into the wood.

(6) With neat diagrams, describe an experiment to show

that pressure increases if the surface area is decreased,

keeping the applied force the same.

Ans. Take a brick measuring 20 cm_10 cm_5 cm. Take some

clay in a glass trough. Add water to it and knead it into a soft dough.

Place the brick on the dough with one of its faces measuring

20 cm_10 cm in contact with the dough. Observe how deep the

brick penetrates into the dough. [Fig. 3.1 (a)]

(a)        (b)

Fig. 3.1 : (Schematic diagrams)

Clean the brick and place it on the dough with one of its faces

measuring 10 cm_5 cm in contact with the dough. Observe how

deep the brick penetrates into the dough.

You will find that the brick penetrates deeper in this case than

that in the first case. [Fig. 3.1 (b)]

In the first case, the weight of the brick acts on a surface area of

200 sq cm.

In the second case, the weight of the brick acts on a surface area

of 50 sq. cm.

This shows that pressure increases if the surface area is

decreased keeping the applied force the same.

* (7) Why are the skis that are used to slide over snow long

and broad?

Ans. The pressure produced by a given force depends on the

area of the surface on which the force acts. The greater the surface

area, the less is the pressure produced. The skis used to slide over

snow are long and broad so that the area is increased and hence the

pressure is decreased. This makes it easier to slide over snow.

* (8) Why do knives and blades have sharp edges ?

Ans. The pressure produced by a given force depends on the

area of the surface on which the force acts. The less the surface

area, the greater is the pressure produced. Knives and blades have

sharp edges so that the area is decreased and hence the pressure is

increased. This makes cutting an object easier.

(9) Why does a needle have a sharp point ?

Ans. The pressure produced by a given force depends on the

area of the surface on which the force acts. The less the surface

area, the greater is the pressure produced. A needle has a sharp

point so that the area is decreased and hence the pressure is

increased. This makes the action of piercing easier.

Answers to the questions given on page 27 of the textbook

* (1) Why do school bags have broad shoulder straps ?

Ans. The pressure produced by a given force depends on

the area of the surface on which the force acts. The greater the

surface area, the less is the pressure produced. School bags

have broad shoulder straps so that the weight of the bag is

distributed over a large surface area thereby decreasing the

pressure on the shoulder of a student carrying the bag.

* (2) Why do porters place the load they have to carry,

on a roll of cloth rather than directly on their head?

Ans. The pressure produced by a given force depends on

the area of the surface on which the force acts. The greater the

surface area, the less is the pressure produced. Porters place

the load they have to carry, on a roll of cloth rather than

directly on their head to distribute the load over a larger area.

This decreases the pressure on their head and neck.

 (10) Describe an experiment to show that the water in a

bottle exerts pressure on the walls of the bottle.

Water

Plastic

bottle

Balloon

Fig. 3.2 : Effect of pressure

Ans. As shown in the figure,

pierce a hole into the side of a plastic

bottle and fit a narrow glass tube,

1 cm long, into it. (The hole can be

made with a hot iron nail.) Fix a

rubber balloon on the mouth of the

tube.

Pour water slowly in the bottle.

You will find that as the water

collects in the bottle, the balloon

bulges and its bulging increases as

the water level in the bottle rises.

This shows that the water exerts pressure on the wall of the bottle.

In this case there is no change in the area of cross section of the

tube. As the water level rises, the mass of the water in the bottle

increases. Due to the corresponding increase in the weight (force),

pressure increases. Hence, the bulging of the balloon increases.

(11) Give two examples to show that air (or a fluid

substance) exerts equal pressure in all directions.

Ans. (1) When air is filled in a balloon, it acquires its

characteristic shape such as round or oval. (2) When a bicycle tube

is filled with air, it acquires its characteristic (tube-like) shape

throughout. This shows that air (or a fluid substance) exerts equal

pressure in all directions.

(12) Describe an experiment to show that fluids flow from

a region of higher pressure to a region of lower pressure.

Ans. As shown in the figure, connect two plastic bottles

slightly above their bases by a rubber tube and a clamp. The clamp

Rubber tube

with a

clamp

(Tight)

Rubber tube

with a

clamp

(Loose)

(a) Starting level (b) Final level

is initially tight. Now, fill one of the bottles completely with water

and the other slightly above the rubber tube.

Make the clamp loose and observe the water levels in the

bottles continuously. You will find that water flows from higher

level (a region of higher pressure) to lower level (a region of lower

pressure) and the flow stops when the levels (pressures) become

equal.

This shows that fluids flow from a region of higher pressure to a

region of lower pressure.

Answers to the questions given on page 29 of the textbook

* (1) Why is a fountain of water seen rising out of a

leaky pipe ?

Ans. The pressure of the water flowing in a pipe is greater

than that of the air outside. Hence, if a pipe is leaky (with a

hole on the upper side) water rushes out through it forming a

fountain.

* (2) When does river water start flowing ?

Ans. River water starts flowing when there is a difference

in water levels (and hence a difference in pressure) along the

length of the river.

(13) Explain the term ‘atmospheric pressure’.

Ans. We are surrounded by the atmosphere made of nitrogen,

oxygen, water vapour and other gases. It extends to hundreds of

kilometres from the ground. The mass of the atmosphere, motion of

molecules of gases and the earth’s gravitational force on the

molecules gives rise to pressure. It is called atmospheric pressure.

If we consider an area of 10 cm_10 cm (_100 cm_) on the

ground, the mass of the corresponding column of atmosphere above

it is almost 1000 kg. The resulting pressure (atmospheric pressure)

is nearly 100000 newton per square metre.

Answers to the questions given on page 30 of the textbook

* (1) Why do our ears ache when we travel by an

aeroplane?

Ans. When an aeroplane descends at a high speed, there

is an increase in air pressure. This increases the pressure on

the ear drum. Hence, our ears ache.

* (2) How is land breeze formed? OR

Why and in which direction do land breezes blow?

Ans. After sunset, the land near the sea gets cooled faster

and to a greater extent as compared to the sea water. Hence,

the air over the land cools quickly. The sea water does not cool

quickly. Hence, the air over the sea remains warm and rises.

Therefore, the air pressure over the land becomes greater than

that over the sea. As a result, a breeze from the land towards

the sea is produced. This breeze is called a land breeze.

(14) How is sea breeze formed?

Ans. During day time, air over the land near the sea gets

heated up faster and to a greater extent than the air over the sea. As

a result, the warm air over the land becomes lighter and rises.

Hence, the air pressure over the land becomes less. Hence, the air

over the sea blows towards the land. This breeze is called a sea

breeze.

(15) Why is there a breeze on the beach in the evening?

Ans. For reference, see the answer to Q. above (14).

(16) Explain why a person may bleed from the nose when

at a great height above the sea level.

Ans. The pressure exerted by the blood in blood capillaries is

slightly more than the atmospheric pressure and acts in a direction

opposite to that of the atmospheric pressure. Atmospheric pressure

decreases with height and at a great height above the sea level, it is

very low. As a result, there arises a difference in the internal and

external pressures on the walls of the cells and blood capillaries. If

the difference is large, it causes the cell wall and the blood

capillaries to burst. Thus, the capillaries in the nose (and ear) burst

causing them to bleed.

(17) Why are we not crushed even though very large

atmospheric pressure acts on us ?

Ans. Our body is made up of cells which contain fluids. These

fluids exert pressure from within. Blood capillaries also exert

pressure. The pressure of blood in our body is slightly greater than

the atmospheric pressure and acts in a direction opposite to it.

Therefore, we are not crushed even though a very large atmospheric

pressure acts on us.

Steam and air

Can Atmospheric

pressure

Steam

(18) When a rubber sucker is pressed onto a flat glass

surface, it sticks tightly on the surface. Why? You need a large

force to separate it from the surface. Why?

Ans. When a rubber sucker is pressed onto a flat glass surface,

practically all the air between the surfaces of the sucker and the

glass is pushed out. The air pressure there becomes much less than

the atmospheric pressure. Hence, the sucker sticks to the glass due

to the external atmospheric pressure.

The atmospheric pressure is about 100000 newton per square

metre. It is very large. Hence, to work against it to separate the

sucker from the glass, a large force is needed.

(19) Describe an experiment to demonstrate that the

atmosphere exerts pressure on an object equally in all

directions.

Fig. 3.4 : Atmospheric pressure

Ans. Take an empty

can made of a thin sheet of

metal (tin). Pour a small

quantity of water in it. Heat

the can (keeping the lid

open) until water begins to

boil. When the steam starts

coming out of the opening,

most of the air in the can is

pushed out and steam

occupies the space of air.

Now, stop heating; put the lid on the can and screw it tightly. Allow

the can to cool. As the steam in the can condenses into water, the

can is gradually crushed. As the steam condenses into water,

partial vacuum is created in the can, i.e., the pressure of the steam,

air and water in the can becomes far less than that of the

atmosphere outside. Therefore, the can is crushed under the external

pressure, i.e., the atmospheric pressure. This shows that the

atmosphere exerts pressure on an object equally in all directions.

(20) Name the scientist who invented a pump to suck out

the air from a closed container.

Ans. Otto von Guericke (a German scientist in the 17th

century) invented a pump to suck out the air from a closed

container.

tube

Rubber bulb

(21) Describe a simple experiment to demonstrate

atmospheric pressure.

Cardboard

Atmospheric pressure

Water

Fig. 3.5 : Atmospheric

pressure

Ans. Fill a glass completely with

water (to its brim) and cover it with a

flat and stiff card paper. Holding your

palm on the card, turn the glass upside

down and take the palm away from the

card.

You will find that the water does

not spill. The atmospheric pressure on

the card (acting upward) is greater than

the pressure of the water in the glass

(acting downward). Hence, the water in

the glass does not spill.

Answers to the questions given on page 32 of the textbook

* (1) Why does a cool breeze blow in the afternoon on

the seashore ?

Ans. For reference, see the answer to Q. 1 (14).

* (2) Why do we pucker our lips when blowing out a

candle ?

Ans. We pucker our lips when blowing out a candle. This

increases the pressure of the air rushing out of the mouth due

to a decrease in area. Therefore, the escaping air has high

velocity and consequently the candle is blown out.

(22) Explain the working of an ink dropper.

Fig. 3.6 : Ink dropper

Ans. An ink dropper consists of a tube of

glass or plastic, with one end tapering to a

narrow opening and the other end fitted with

a small rubber bulb. When the narrow open

end is dipped in the ink and the rubber bulb is

pressed, some air in the tube escapes through

the open end. This reduces the air pressure

inside the dropper.

On releasing the bulb, the atmospheric pressure on the ink

pushes the ink into the dropper.

3. ATMOSPHERIC PRESSURE 37

The dropper is then taken out and its open end is held over

the open barrel of the pen. The bulb is then pressed so that the ink

in the dropper enters the pen.

(23) Explain the statement : An ink dropper is a kind of

pump.

Ans. For reference, see the answer to Q. (22).

When an ink dropper is filled with ink, ink continues to enter

the dropper till the pressure inside and the pressure of the outside

air near the opening of the tube become equal. Later, the ink comes

out of the dropper only when the rubber bulb is pressed. This shows

that an ink dropper is a kind of pump.

* (24) Why does ink not spill out of an ink dropper?

Ans. Ink does not spill out of an ink dropper because the

pressure inside and the external atmospheric pressure are equal.

Answers to the questions given on page 33 of the textbook

* (1) Why is the opening of a dropper very narrow?

Ans. The pressure produced by a given force is inversely

proportional to the area of the surface on which the force acts.

The opening of a dropper is very narrow. Hence, its area of

cross section is very small. As a result, even if the dropper

has a small amount of ink in it, its pressure can equal the

atmospheric pressure. As the opening is narrow, it is easier to

transfer the ink to the pen. The possibility of ink spilling is

very low.

* (2) What is the characteristic of the cap of eye drop

bottles ?

Ans. The cap of an eye drop bottle is fitted with a dropper.

(25) Explain the working of a spray pump.

Handle

Piston with a rod

Lid

Cylinder

Narrow tube

Fig. 3.7 : Spray pump

Ans. As shown in the figure, a

spray pump consists of a cylinder

made of plastic or metal fitted with

a piston. One end of the cylinder

is in the form of a narrow tube.

The snugly fitting piston can

slide in and out smoothly. The

rod connected to the piston

passes through a hole in the centre of the lid and has a handle at the

other end.

When the tip of the narrow tube is dipped in water (or any other

liquid) and the piston is pushed towards the tip, up to the bottom,

practically all the air in the cylinder escapes through the tube,

reducing the pressure. When the piston is moved up, the water rises

in the part of the cylinder below the piston due to the atmospheric

pressure. Finally, the inner pressure equals the atmospheric

pressure and no more water enters in or comes out. To spray the

water, the tube is taken out and the piston is moved towards the

opening of the tube. As the inner pressure is now greater than the

atmospheric pressure, the water gushes out of the narrow opening

of the tube.

Answers to the questions given on page 34 of the textbook

* (1) What is the difference between the ink dropper

and the spray pump?

Ans. In the ink dropper, the rubber bulb is used to reduce

the inner pressure before filling the ink in the dropper. In the

spray pump, the piston is moved towards the opening of the

tube to reduce the inner pressure before filling the water in the

pump.

In the ink dropper, while taking ink in, the pressure on the

bulb is reduced. In the spray pump, while taking water in, the

piston is moved away from the opening of the tube.

In the ink dropper, the bulb is pressed to fill ink in the pen.

In the spray pump, the piston is moved towards the opening of

the tube to spray water.

* (2) How does the doctor’s syringe work?

Ans. For reference, see the answer to Q. 25.

The tip of a syringe is fitted with a very fine and hollow

needle. The required quantity of medicine can be taken in the

syringe with the help of the piston. The medicine can then be

injected into the body of a patient using the needle and the

piston.

Q. 2 Give scientific reasons :

(1) The tiles are placed over a slushy patch of ground to

help cross it.

Ans. (1) Tiles have greater area than the area of our feet.

(2) The weight of the person crossing the slushy patch is exerted

over a large area of the tiles. (3) Therefore, there is a decrease in the

3. ATMOSPHERIC PRESSURE 39

pressure and hence the tiles do not sink more in the slushy patch of

ground. This helps to cross the slushy patch of ground.

[Note : If there were no tiles, the feet will come in direct

contact with the slushy ground. The area of the feet being less, the

weight of the person will act over a smaller area. Therefore, there

will be more pressure and hence the feet will sink into the slushy

ground.]

(2) An ST bus has four rear wheels.

Ans. (1) The weight of the passengers and their luggage exerts

a large force on the rear wheels of the bus. This creates a large

pressure on the rear wheels. (2) To withstand this large force and

reduce the pressure created by the force, the area of the rear wheels

should be increased. This is done by providing two wheels on each

side of the rear portion of the bus.

(3) Drawing pins have flattened heads.

Ans. (1) The head of a drawing pin is flattened and the other

end is pointed. (2) When enough force is applied to the head of the

pin, the pressure due to the force on the pointed end increases

tremendously and the pin can be easily inserted in the drawing

board. (3) When we press the flattened end, the force applied

spreads over a larger area. This reduces the pressure of the reaction

force acting on the thumb. Hence, the thumb is not injured. (4) If

the head of the pin is sharp, then the pressure due to the force would

be more and hence the pressure of the reaction force would also be

more and the sharp end would prick the thumb causing injury.

Q. 3 Answer the following questions in one sentence

each :

(1) Name the physical quantity which has unit newton per

square metre.

Ans. The unit of pressure is the newton per square metre.

(2) If the force is kept constant and the surface area on

which the force acts is reduced, will the pressure increase or

decrease?

Ans. If the force is kept constant and the surface area on

which the force acts is reduced, the pressure will increase.

40 NAVNEET GENERAL SCIENCE DIGEST : STANDARD VIII

(3) Keeping the surface area constant, if the applied force

is reduced, will the pressure increase or decrease ?

Ans. Keeping the surface area constant, if the applied force is

reduced, the pressure will decrease.

(4) What is the mass of the column of atmosphere standing

on a 100 cm_ area on the earth’s surface ?

Ans. The mass of the column of atmosphere standing on a 100

cm_ area on the earth’s surface is almost 1000 kg.

(5) State whether the sea breeze flows from the land

towards the sea or from the sea towards the land.

Ans. The sea breeze blows from the sea towards the land.

Q. 4 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

* (1) The pressure of air inside an inflated balloon is equal to the

atmospheric pressure.

* (2) Fluids always flow from higher pressure to lower pressure.

* (3) If area is reduced, pressure is reduced.

* (4) Standing on a cane chair increases the possibility of the

cane breaking due to increased pressure.

(5) Pressure_force_area

(6) If the force acting on a given surface area is increased, the

pressure increases in the same proportion.

(7) The unit of force is the newton.

(8) The unit of pressure is the newton per metre.

Ans. (1) True. (2) True. (3) False. If area is reduced, pressure

increases. (4) True. (5) False. Pressure_force / area. (6) True.

(7) True. (8) False. The unit of pressure is the newton per square

metre.

Q. 5 Fill in the blanks :

*1. (1) The unit of pressure is the ……… .

(2) Ink rises into the ink dropper because of the………pressure

acting on the surface of the ink in the bottle.

(3) When the piston of a pump is pulled up, the pressure inside

……… .

(4) The air pressure inside our body is equal to the ……… .

Ans. (1) newton per square metre (2) atmospheric

(3) decreases (4) atmospheric pressure.

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3. ATMOSPHERIC PRESSURE 41

2. (1) Pressure_………/ area.

(2) Due to a difference in ………, a liquid can be transferred

from one container to the other.

(3) ……… brakes are used in trucks, buses, trailers, etc.

(4) The ……… applied on unit area is called pressure.

Ans. (1) force (2) pressure (3) Air (4) force.

Q. 6 Choose the correct alternatives and complete the

following statements :

(1) Pressure_……… .

(a) force_area (b) force / area

(c) mass / area (d) area / force

(2) The unit of pressure is the ……… .

(a) newton (b) newton per metre

(c) newton per square metre (d) newton.metre_

(3) The newton is the unit of ……… .

(a) velocity (b) pressure (c) mass (d) force

(4) Keeping the surface area constant, if the applied force is

doubled, the pressure ……… .

(a) becomes double (b) remains the same

(c) becomes four times (d) becomes half

Ans. (1) force / area (2) newton per square metre (3) force

(4) becomes double.

* Q. 7 Match the following :

‘A’ ‘B’

(1) Fluid substances

(2) Sprinkle irrigation

(3) Blunt weapon

(4) Sharp weapon

(a) Greater pressure

(b) Equal pressure in all

directions

(c) Pressure of water

(d) Less pressure

Ans. (1) Fluid substances – Equal pressure in all directions

(2) Sprinkle irrigation – Pressure of water (3) Blunt weapon– Less

pressure (4) Sharp weapon– Greater pressure.