Pressure Class 8 Questions and Answers Notes Basic Science Chapter 3 Kerala Syllabus

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Class 8 Basic Science Chapter 3 Pressure Question Answer Notes

Class 8 Basic Science Chapter 3 Notes Kerala Syllabus Pressure Question Answer

Pressure Class 8 Questions and Answers Notes

Let’s Assess

Question 1.
Give reasons.
• Heavy vehicles have more tyres.
• Some passengers experience nosebleeds when airplanes fly at high altitudes.
• The size of the bubbles rising from the bottom of an aquarium increases gradually.
• The foundation of buildings is made wider.
Answer:
• Heavy vehicles are provided with more tyres to increase the surface area of contact with the surface of the road so that it reduce the pressure on each tyre and on the surface of the road and ensures smooth motion.
• As the altitude increases the atmospheric pressure decreases. This lower atmospheric pressure leads to nosebleeds in some passengers.
• At the bottom of the aquarium, the pressure is high. For a bubble coming up the pressure experienced is less. Hence the volume of bubble increases.
• As the foundation is made wider, the pressure exerted by the building on the ground decreases because surface area of contact between building and ground increases. Thus in turn makes the building stronger and safer.

Question 2.
Which of the bottles below shows the shape of rising bubbles correctly? Explain why.

Answer:
Bottle C shows the correct shape.

Liquid pressure is greater at the bottom. For a bubble coming up pressure decreases. Hence the size of the bubbles increases as it moves from bottom to top.

Question 3.
Observe the picture below. Which of the following jars containing the same liquid, will experience the highest pressure at the bottom? Explain why.

Answer:
All the three jars containing the same liquid will experience the same pressure. Liquid pressure depends on the depth and the density of the liquid. Since the liquids are same liquid pressure is not dependent on density here. Liquid pressure is same here as the depth of the liquid (or height of liquid column) is same in all the jars.

Question 4.
Cooking is faster in a pressure cooker than in an open vessel. Explain why.
Answer:
When we cook food in a pressure cooker, the pressure inside it increases as the temperature increases. The boiling point of water is raised by this increased pressure inside it. This allows food to be cooked at a higher temperature and makes cooking faster than in an open vessel.

Basic Science Class 8 Chapter 3 Question Answer Kerala Syllabus

Question 1.
Let’s now have a look at the figure given below, in which the same weight is applied in two different ways.

A brick is placed vertically and then horizontally on a sponge (Fig(a)). In both cases, does the weight of the brick remain the same? Are the compressions on the sponge equal?
Answer:
The weight of the brick remain the same in both cases. The compressions on the sponge is not equal.

Question 2.
The weight of the child remains the same whether they stand or lie on the mattress (Fig.(b)) as shown above. Yet, the mattress is compressed more while standing. What could be the reason for this?
Answer:
The mattress is compressed more while standing because the surface area in contact is less in this case.
Activity

Fix a single nail on the cardboard as shown in the figure and place a balloon on top of it. Place a small weight on the balloon. Observe what happens to the balloon. Now, nail multiple pins very close to each other as shown in the second figure. Place the same weight on top of the balloon. Observe the result.

Observation
When weight is placed on the balloon, placed on a single nail it bursts. When the same weight is placed on the top of the balloon, placed on the top of multiple pins it will not burst.

Question 3.
Record the observations from the above activity in the table below.

Answer:

Question 4.
Analyze the table.
Answer:
Even when the same force is applied in each case, the results vary depending on the change in surface area in contact.

Question 5.
What is pressure?
Answer:
Pressure is the force acting normally per unit area.

Question 6.

Who will win the race? Write your opinion.
Answer:
Duck will win the race. Hen cannot move at the same speed as that of duck in mud.

Question 7.
What could be the reason?
Answer:
Webbed feet of duck makes more surface area of contact with the mud. Thus the pressure experienced on its feet is less and it can walk over mud easily. But the hen’s feet make less surface area of contact with the mud and hence experience more pressure which makes it difficult for it to move over the mud.

Question 8.
See the figure of two concrete slabs placed in sand.

Each of them is made of cubes weighing 18 N fixed together. The area of one side of the cube is 0.36 m²


Referring to the figures, complete the following table.
Answer:

Measurements	Slab 1	Slab 2
Force experienced on the sand	18 N	18 N
Surface area of contact	0.36 × 4 = 1.44 m2	0.36 m2
Force experienced per unit area	18/1.44 = 12.5 N/m2	18/0.36 = 50 N/m2

The weight of the slab acts normally on the sand. This is called thrust.
The force acting normally on a surface is called thrust. The force acting normally per unit area is pressure.
If pressure is denoted by P, thrust by F and area by A, pressure = \(\frac{\text { Thrust }}{\text { Area }}\) P = \(\frac{F}{A}\)
Unit of pressure = \(\frac{\text { Unit of thrust }}{\text { Unit of area }}\)
The SI unit of pressure is pascal. 1 pascal = 1N/m²

Question 9.
Now, find out the pressure exerted by the above slabs, on the sand.
Answer:
Pressure exerted by slab 1 = 12.5 N/ m² or 12.5 Pa
Pressure exerted by slab 2 = 50 N/ m² or 50 Pa

Question 10.
A concrete block weighing 100 N is placed on sand in three different ways as shown in the figure below.

a) In which position will the highest pressure be exerted on the sand?
Answer:
Higher pressure will be exerted on the sand when the brick is placed vertically (position 3)

b) Which position will cause the concrete block to sink deeper into the sand? Why?
Answer:
The third position (vertical) will cause the concrete block to sink deeper into the sand. It is because the surface area of contact between the concrete block and sand is less here.

When thrust remains constant, the pressure is inversely proportional to the surface area. This means, when the surface area increases, the pressure decreases and vice versa.

Question 11.
Can you explain the reason why a lorry would not get stuck in the mud if a wooden plank is put.
Answer:
When a wooden plank is put over mud, the surface area of contact between the lorry and the mud increases, so the pressure experienced on the mud becomes less, so that the lorry will not get stuck in mud.

Question 12.
Try to lift your school bag by tying it with a thin twine. How do you feel?
Answer:
It is very difficult to lift a school bag by tying it with a thin twine because pressure felt on the hands is very high.

Question 13.
What if you lift it using a wide strap as shown in the figure? Can you explain the difference by relating its area to pressure?

Answer:
When the bag is lifted using a wide strap the surface area of contact between the bag and the hand is more, so the pressure felt on the hands is less.

But when thin strap is used, the surface area of contact between the bag and the hand is less, so the pressure felt on the hands is more. This makes it difficult to lift the bag.

Question 14.
Similarly, explain the following situations in the figure. Suggest more such situations.

Answer:

• The edge of a knife is made thinner – Thin edges have less surface area of contact which helps to exert more pressure on the objects to be cut.
• The wheels of bulldozers are connected by wide chains -Wide chains make more surface area of contact with the mud to exert less pressure over it. This helps to prevent the wheels getting stuck in the mud and ensures its smooth motion.
• A person lying on a bed of nails will not bleed- As the bed of nails make large surface area of contact with the person, the pressure exerted on the person is less. So person will not bleed.
• The basement of dams is made wider -Basements are made wider to increase the surface area of contact betw een walls and basement and thus to withstand the high pressure exerted by water stored in dams.

Question 15.
Do only solids exert pressure?
Answer:
No

Question 16.
Can’t liquids and gases exert pressure?
Answer:
Yes, Liquids and gases also exert pressure.
Activity
Tie a polythene cover tightly around your hand as shown in the figure. Dip your hand into a bucket of water.

Observation: The cover sticks to your hand.
Inference: This happens due to the pressure exerted by the water on the polythene cover.
Thus we can understand that, just like solids, liquids can also exert pressure.

The normal force exerted by a liquid is called thrust. The thrust that a liquid exerts per unit area is called liquid pressure. Liquids exert pressure on all sides of the container.

Question 17.
If the force of water jetting out is greater, wouldn’t the pressure also be.greater?
Answer:
Yes. When the force of water jetting out is greater the pressure also will be greater.

Question 18.
What happens to the water stream when the water level decreases?
Answer:
When the water level decreases the force of water jetting out becomes less and hence the pressure also will be lesser.

Question 19.
What can be inferred from this?
Answer:
As the force decreases due to decrease in water level pressure also decreases and pressure depends on the depth of the liquid.

Pressure increases as the depth in a liquid increases.

Activity
Attach a syringe on a board as shown in the figure below.

Attach one end of the I.V. set tube to the hub of the syringe. Attach the piston of the syringe to the bottom end of a pen barrel. Arrange it so that the pen barrel deflects when the piston moves. Now fix a thin stick to the top end of pen barrel as a pointer. Attach the other end of the I.V. set to the plastic bottle. Fill the plastic bottle with water and place it at a higher level than the syringe.

Question 20.
Raise the bottle. The force applied by the piston will change according to the pressure of the water. Now’ you can see the thin stick deflecting according to the force exerted in the piston, right?
Answer:
Yes. We can see the thin stick deflecting according to the force exerted in the piston.

Question 21.
Raise the bottle further. What do you observe? What is the reason?
Answer:
The thin stick deflects more. When the bottle is raised further, height of the liquid column increases and pressure of water increases and the force applied by the piston also increases.

Question 22.
Repeat the experiment using salt solution instead of water. Why does the thin stick deflect more?
Answer:
Water and saline water have different densities. Density of saline water is higher than that of water. When density of a liquid increases, pressure exerted by it increases. So the force exerted on the piston by saline water is more. That is why the piston moves more, causing greater deflection of the thin stick.

The density of a liquid influences its pressure. If the density of a liquid mercases, the pressure also increases.

The instrument used to measure liquid pressure is called a manometer.

Question 23.
Do you see any difference in the level of water in the tube?
Answer:
Yes . When slight pressure is applied the level of water in tube rises up.
Apply more pressure. You can see a difference in water levels also.

Question 24.
Now take different solutions in the beaker and measure the pressure at different levels. Write your observations in the table below.

Answer:

Position of the funnel	Difference in the water levels of U-tube/pressure
On the surface	In water	In saline water	In kerosene
Midway in the beaker	12 cm	12 cm	12 cm
At the bottom of the beaker	18 cm	20 cm	16 cm
Position of the funnel	24 cm	26 cm	20 cm

Question 25.
Analyse the table.
a) In which position is the pressure highest?
Answer:
Pressure is highest at the bottom of the beaker.

b) Which liquid exerts the highest pressure?
Answer:
Saline water exerts the highest pressure.

Question 26.
Fill a deep vessel with water. Dip a straw into the water and blow gently. Do bubbles come up?

Answer:
Yes, bubbles come up.

Question 27.
What happens to their size when they reach the surface?
Answer:
Size of the bubble increases.

Question 28.
Explain their change in size.
Answer:
The increase in size of bubble when it reach the surface is due to the decrease in pressure.

Question 29.
Explain why dams are built with a wider base.
Answer:
Dams are built with a wider base to withstand the pressure exerted by the water stored in it. When there is a wider base, the surface area of contact between water and the base of the dam is more and so pressure exerted decreases.

Question 30.
Can you think of more situations in daily life where liquid pressure is experienced?
Answer:

• Water tanks at home are placed at a height to exert liquid pressure to the pipelines in the building.
• Oil tankers are made of thick metals to withstand the pressure of huge volume of oil.
• When a bucket of water is carried on head pressure is felt on head.

Factors influencing gas pressure
Question 31.
Inflate a balloon at its maximum. What happens when the balloon is inflated further? Will it eventually burst?
Answer:
The balloon bursts as it fails to withstand the air pressure inside.

Question 32.
Find the change in the number of gas particles, in figure shown below, when more air is filled. What can be inferred from this?

Answer:
When more air is filled, the number of gas particles increases and the gas pressure also increases.

Gas pressure depends on the number of particles. As the number of particles increases, gas pressure also increases.

Question 33.
Fill the same quantity of air into two balloons of different sizes. Now the number of gas particles in both . balloons is equal, right? Which of the balloons has more pressure inside? What can be inferred from this?
Answer:
As the same quantity of air is filled into two balloons the number of gas particles in both balloons is equal. But the balloon with smaller size has more pressure as it has less volume. When the volume is less, the gas particles has less space to move around. This leads to the frequent collision with the walls of the balloon and hence more pressure is exerted.

Gas pressure depends on its volume.

Question 34.
Leave an inflated balloon in the sunlight. What do you observe? What is the change in the pressure when the air inside gets heated up?
Answer:
When the air inside gets heated up, the pressure inside the balloon increases and it bursts after some time.

Gas pressure depends on its temperature.

When we cook food in a pressure cooker, the pressure inside increases as the temperature increases.

Question 35.
List the factors influencing gas pressure.
Answer:

• Number of particles
• Volume
• Temperature

Question 36.
Is atmospheric pressure the same everywhere on the Earth? Observe the figure.

Which book will experience the highest pressure? Why?
Answer:
No, the atmospheric pressure is different at different places on earth. The book at the bottom experiences the highest pressure as it experiences the atmospheric pressure and the combined weight of all the books stacked above.

Question 37.
Similarly, we know that atmospheric pressure is the weight of air column experienced per unit area. So, what is the change that occurs in atmospheric pressure as altitude increases? Note your inferences.
Answer:
Atmospheric pressure decreases as height from the Earth’s surface increases.

Atmospheric pressure at sea level is known as standard atmospheric pressure. It is defined as the weight of a mercury column that is 0.76 m high, with a unit cross sectional area, exerting a pressure of 1 atm. The instrument used to measure atmospheric pressure is a barometer.

Question 38.
Take an aluminum can and fill it with hot water. Pour out the hot water and immediately seal the can. Immerse the can in cold water. What do you observe?

When the can cools down, the pressure inside the can decreases. The external atmospheric pressure is strong enough to crush the can.

Question 39.
Calculate the force exerted by atmospheric pressure on a table surface of area 1 m². Atmospheric pressure is 101325 Pascal.
Answer:
Force = Pressure × Area
= 1 Pa × 1 m²
= 101325 N
This is equivalent to the weight of an object with a mass of 10339 kg. If such a large force is exerted on the table, it doesn’t collapse. It is because the pressure is applied from all sides on it and so the forces get balanced.

Gas pressure is exerted in all directions, similar to that of liquids.

Activity
Fill a glass with water and cover it with a cardboard. Hold the cardboard with your hand and turn the glass upside down. Remove your hands slowly.

Question 40.
Does the water fall?
Answer:
The force due to atmospheric pressure can hold the entire weight of the water in the glass. So the water does not fall down.
The equilibrium of pressure between gases and liquids is a common phenomenon occurring in nature.

Question 41.
What happens to the candle flame?
Answer:
The candle flame goes out.

Question 42.
What happens to the air pressure inside the glass?
Answer:
Air pressure inside the glass decreases.

Question 43.
What is the change in the water level?
Answer:
Water level inside the glass rises.
The water level remains stable when the pressure inside the glass and the atmospheric pressure outside the glass are in equilibrium.

Question 44.
Gently pull back the piston of a syringe and seal the opening with your hand. Then pull the piston all the way back and release. What do you see?
Answer:
The piston will go back. When the syringe opening is sealed and the piston is pulled back and released, the piston will move into the syringe barrel. This movement is due to the difference in pressure between the outside and inside of the syringe. The higher outside pressure forces the piston back into the syringe barrel.

Question 45.
Why does the piston go back? Discuss the reason.
Answer:
The piston move back into the syringe barrel, because the pressure inside the syringe is less than the pressure outside. So the higher pressure outside forces the piston back into ehe syring barrel.

Question 46.
Read the following situations and explain them based on atmospheric pressure.
• Rubber suckers stick to smooth surfaces.
• Mountain climbers often experience nosebleeds at high altitudes.
• Holes are made on the injection bottle with a needle during a drip injection.
• Passengers travelling uphill in vehicles on a ghat road experience ear pain.
Answer:
• When a rubber sucker is pressed on a smooth surface, it pushes the air out. This creates low pressure inside. The higher air pressure outside pushes it onto the smooth surface making, it stick.
• Mountain climbers often experience nosebleeds at high altitudes because as altitude increases atmospheric pressure decreases.
• For the smooth flow of liquid there should be air flow. In the absence of a hole, air cannot enter the bottle and the flow stops. Making a hole lets air into the injection bottle and keeps the pressure balanced, ensuring continuous flow.
• As the altitude increases the atmospheric pressure decreases. But the pressure inside the ears remain the same for a while and this difference in pressure causes a feeling of pain in the ears.

Class 8 Basic Science Chapter 3 Question Answer Extended Activities

Question 1.
Conduct a study on the topic ‘Rain and Pressure’ and prepare a project report.
Answer:
An example of a project report is shown below.
Title: Rain and pressure

Introduction:
Rainfall is one of the most important phenomena. It plays a major role in agriculture, environment and many more day to day activities. One of the important factors that influence weather patterns is pressure. Here we explore the relationship between pressure and rain.

Main points to be included in the content

• The continuous movement of water through the phases of evaporation, condensation, precipitation, and collecting is known as the water cycle.
• Water vapour, which rises into the atmosphere, is created when the Sun heats water in lakes, rivers, and seas.
• In regions of low pressure, the air is generally less dense and rising.
• As warm air rises it cools down.
• When air cools, the vapour in it turns back into small liquid droplets or ice crystals to form clouds. This process is called condensation.
• Precipitation is the term for the water that returns to the earth as rain, snow, or hail when the clouds becomes heavy.
• The cycle then restarts when the water gets collected in lakes, rivers, or underground, with atmospheric pressure being a major factor in causing rain to fall and water vapour to rise.
• When the air pressure is high it means the air is generally more dense and sinking.
• Sinking air warms up.
• Warm air can hold more water vapour without condensing into droplets.
• So high pressure areas are usually associated with sunny weather and clear skies.
  (You can also refer to weather reports in newspapers and include points from it, add flowcharts and diagrams, various weather study reports)

Conclusion
Pressure and rainfall are interrelated.

Question 2.
Attach a pipe to each end of a plastic bottle, ensuring it airtight. One pipe should be longer and the other shorter. Place one end of the pipe in a bucket filled with water and placed at a highest level. Place the shorter pipe in another vessel. Squeeze and release the plastic bottle two or three times. Does the water from the bucket flow into the lower vessel? State the reason for this.
Answer:
Yes, the water from the bucket will flow into the lower vessel. When the bottle is squeezed, the pressure inside it increases. This higher pressure pushes air into the water through the longer pipe and forces water to rise up through it and then water is pushed down the longer pipe and into the shorter pipe, which then flows into the lower vessel due to gravity.

Pressure Class 8 Notes

Class 8 Basic Science Pressure Notes Kerala Syllabus

• The force acting normally on a surface is called thrust. The force acting normally per unit area is pressure.
• If pressure is denoted by P, thrust by F and area by A, Pressure = \(\frac{\text { Thrust }}{\text { Area }}\) P = \(\frac{\mathrm{F}}{\mathrm{~A}}\)
• Unit of pressure = \(\frac{\text { Unit of thrust }}{\text { Unit of area }}\). The SI unit of pressure is pascal. 1 pascal = 1N/m²
• When thrust remains constant, the pressure is inversely proportional to the surface area. This means, when the surface area increases, the pressure decreases and vice versa.
• The normal force exerted by a liquid is called thrust. The thrust that a liquid exerts per unit area is called liquid pressure.
• The factors affecting liquid pressure are
  • Height of the liquid column (h)-As height of the liquid column increases, liquid pressure increases.
  • Density of the liquid (d)-As density of liquid increases, liquid pressure increases.
• The instrument used to measure liquid pressure is called a manometer.
• The force exerted by a gas normally on a unit area is gas pressure.
• Factors influencing gas pressure.
  • Number of particles-As number of particles increases, gas pressure increases.
  • Volume-As volume increases, gas pressure decreases.
  • Temperature-As temperature increases, gas pressure increases.
• Atmospheric air exerts pressure on objects.
• The weight of air column per unit area on the earth’s surface is atmospheric pressure. The unit of atmospheric pressure is ‘bar’.
• Atmospheric pressure at sea level is known as standard atmospheric pressure. The instrument used to measure atmospheric pressure is a barometer.
• Gas pressure is exerted in all directions, similar to that of liquids.

INTRODUCTION

The concept of pressure is important in science and it is an inevitable part of our day to day life. It explains simple daily life situations like using a sharp knife to cut better, making use of syringes, air pumps to applications like making the base of dams wider and weather predictions. Not only solids, but liquids and gases also exert pressure. This chapter deals with topics like pressure and its measurement, liquid pressure, gas pressure and atmospheric pressure.

PRESSURE AND ITS MEASUREMENT
We have learnt about different types of forces. The earth exerts an attractive force on all objects. That is the weight of the object.

ACTIVITY TO FIND OUT THE FACTORS LIQUID PRESSURE DEPENDS ON
Make a hole at the bottom of a plastic bottle. Close the hole and fill the bottle with water. Open the hole and observe the water jetting out.

Repeat the experiment using bottles of different shapes and sizes.

MAKING A MANOMETER

Fix a plastic tube on a board in a U shape. Fill it with water. Connect a funnel to one end of the tube as shown in the figure. Make a diaphragm with a balloon on the mouth of the funnel. Attach a scale to the board. Now the manometer is ready.
Apply slight pressure with your finger on the balloon.

GAS PRESSURE
Just like solids and liquids, gases also have the ability to exert pressure. As we know, pressure is measured when air is filled into vehicle tyres.

Pressure is the force exerted by the particles of a gas on unit area of a surface. This is due to the collision of gas particles.

The force exerted by a gas normally on a unit area is gas pressure.

ATMOSPHERIC PRESSURE
The atmosphere is a blanket of air surrounding the Earth. The density of air decreases as we go higher.

ACTIVITY TO UNDERSTAND IF ATMOSPHERE CAN EXERT PRESSURE
Place a scale on the table as shown in the figure. Drop an ice cream ball on the scale from a small height. The scale falls down. Place an A4 size paper on the scale. Drop the ball again from the same height.

Now, the scale does not fall down. It is because of the weight of the air above the paper. The atmospheric pressure counteracts the force exerted by the ball.

Atmospheric air exerts pressure on objects.

From the above activity, we understood the influence of atmospheric pressure.

The weight of air column per unit area on the earth’s surface is atmospheric pressure. The unit of atmospheric pressure is ‘bar’.

PRESSURE BALANCE
Activity
Pour water in a flat container. Place a lit candle in it. Cover the candle with a glass.