LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 13 : SOUND

CLASS VIII   |    SCIENCE    |    CHAPTER 13

      notes prepared by subhankar Karmakar

                                                                         

SOUND 

Sound is a form of energy. Sound is that form of energy which makes us hear. A vibrating object always produces sound. Each sound is special to the object which produces it. 

In some cases, the vibrations of a sound producing object are quite large which we can see with our eyes. But in other cases, the vibrations of the sound producing object are so small that we cannot see them easily, we have to feel the vibrations of such an object by touching it gently with the fingers of our hand. 

Like, sound is produced by a vibrating bicycle Bell. Sound is produced when a stretched rubber band vibrates. Sound is produced when a sitar string vibrates. Sound is produced when our vocal cords vibrate. Sound is produced when the air column enclosed in a tube vibrates. Sound is produced when the membrane of a tabla vibrates. Sound is produced when the stretched membrane of a drum vibrates. Sound of a radio or television or speakers is produced by the vibrations of the the cone of the speakers. 

Sound can be produced by the following methods:

1. By vibrating strings (in guitar, sitar etc)

2. By vibrating air columns (in flutes)

3. By vibrating membranes (in tabla, drums)

4. By vibrating plates (in bicycle bells)

Propagation of sound:

When an object vibrates then the air around it also starts vibrating in exactly the same way and carries sound to our ears through the vibrations of its molecules. 

Sound produced by humans:

The human beings produce sound by using the voice box which is called larynx. It is also known as the voice box. The human voice box or larynx contains two ligaments known as vocal cords. The vocal cords act like a kind of strings. Sound is produced by the vibrations of vocal cords. 

The lungs pass a current of air between the two vocal cords. This air makes the vocal cords vibrate and the vibrating vocal cords produce sound. 

When the muscles attached to the vocal cords contract and stretch, the vocal cords become tight and thin, and a sound of high frequency is produced. When the muscles relax, the vocal cords become loose and thick and a sound of low frequency is produced. The vocal cords of a man are about 20 mm long. The vocal cords of a omen are about 5 mm shorter than man. Due to the shorter vocal cords, the frequency or pitch of a woman's voice is higher than that of a man. Small children have very short vocal cords due to which the frequency of each of their voice is also very high. 

SOUND NEEDS A MEDIUM FOR PROPAGATION:

Sound needs a solid, liquid or gas for transmission. Therefore, we can say, sound needs a material medium like solid liquid or gas to travel and be heard. Sound cannot travel through vacuum or empty space, as there is no molecules which can vibrate and carry sound waves in empty space. 

Sound cannot be heard on the surface of moon because there is no Air on the moon to carry the sound waves. The astronauts who land on moon talk to each other through wireless sets using radio waves. 

SPEED OF SOUND

We know sound can travel through solids, liquids and gases. The speed of sound is different in different materials. Sound travels slowest in gases, faster in liquids and fastest in solids. Sound travels at a speed of 340 m/s, in water at 1500 m/s and in Iron 5000 m/s. 

The speed of sound in air is 340 m/s whereas light travels at 300000 m/s. It is due to the very high speed of light that we see the flash of lightning first and it is due to comparative low speed of sound that the thunder is heard a little later. 

STRUCTURE OF HUMAN EAR

The ears are the sense organs which help us in hearing sound. 

• Pinna: The funnel shaped outer part of the ear is called pinna. Sound waves enter our ear through pinna. 

• Ear canal: Pinna is attached to about 2 to 3 centimetre long passage called ear canal.

• Ear drum: Ear canal is attached with a thin elastic and circular membrane called ear drum. 

• Three tiny ear bones: There are three small and delicate bones called Hammer, anvil and stirrup in the middle part of the ear which are linked to one another. The three tiny bones in the middle ear act as a system of levers and amplify sound vibrations coming from the ear drum before passing them on to the inner part called cochlea. 

The free end of stirrup touches the membrane over the oval window. 

Cochlea: The inner part of ear has a coiled tube called "cochlea". One end of cochlea is connected to middle part of ear through the elastic membrane over the oval window. Cochlea is filled with a liquid. The liquid present in cochlea contains nerve cells which are sensitive to sound. The other end of cochlea is connected to auditory nerve which goes into the brain. 

WORKING OF EAR:

The sound waves are collected by the pinna. Then it passes through ear canal and fall on ear drum. It makes ear drum vibrating back and forth, which makes hammer to vibrate. Vibrations then amplified and passes through anvil and stirrup. This amplified vibrations passes through oval window to cochlea. This makes liquids in cochlea to vibrate and generates electrical impulses in the nerve cells. These electrical impulses are carried by auditory nerve to brain and the brain interprets these impulses as sound and thus we get the sensation of hearing. 

LECTURE - 3 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      notes prepared by subhankar Karmakar

                                                                         

FRICTION: A NECESSARY EVIL

Frictional force is very important in our life. Sometimes friction is useful and we want to keep it and sometimes it is harmful and we want to reduce it. 

ADVANTAGES OF FRICTION:

1. Friction enables us to walk without slipping

We are able to walk on ground because friction between the sole of our shoes and ground prevents us from slipping over the ground. Walking on slippery ground is difficult because the frictional force on slippery ground is much less which may not be sufficient to prevent us from slipping.

When we accidentally step on a banana peel thrown on the road we easily slip and fall down.

2. Friction enables a car to move on road without skidding.

The friction between tyres of a car and the road enables a car to move forward on road without skidding. If there were no friction between car tyres and road, then the wheels of car would spin at the same place but the car would not move forward at all the car would stay where it was.

3. Friction enables us to apply brakes and slow down or stop a moving car.

The brakes of a car work by friction the friction between break pad and ud rim prevents the wheel from moving ahead. If there were no friction then once a vehicle started moving it would never stop. 

4. Friction enables us how to write and draw on paper.

We are able to write and make drawings on paper because there is friction between the tip of pencil or pen and paper. 

We cannot write with a pencil on a glass sheet because the glass surface is very smooth due to which the friction between the tip of pencil and glass surface is much less. This friction is not sufficient to rub off black graphite particles from the tip of pencil.

5. Friction enables us to pick up and hold things in our hands.

We can hold a glass tumbler in our hands because of friction between the glass tumbler and our hands. If the outer surface of a glass tumbler is oily or greasy then it becomes difficult to hold it because a film of oil on the outer surface reduces friction.

6. Nails can be fixed in a wall or wood due to friction.

When we hammer a nail into the wall, it is the friction between the surface of nail and wall which holds the nail tightly in the wall. 

(i) nails and screws are also held in wood by friction.

(ii) friction enables knots to be tied in strings.

(iii) Friction enables a person to climb a tree or pole.

(iv) Friction enables a ladder to be leaned against a wall. 

(v) Friction helps in the construction of buildings.

(vi) Friction enables the belts to drive machines in factories. 

7. Friction enables us to light a matchstick.

When we rub a matchstick against the rough side of a Matchbox than friction between the head of matchstick and rough side of matchbox produces heat. This heat burns the chemicals present on the head of matchstick due to which the matchstick lights up. The burning of matchstick would not be possible without friction. 

8. Friction enables us to cut wood with a saw.

We are able to cut wood because there is friction between the the saw blade and log of wood. 

DISADVANTAGES OF FRICTION:

1. Friction wears away the soles of our shoes.

When the soles of our shoes rub against the rough surface of road, then tiny pieces of the soles keep on breaking off slowly due to which the soles of our shoes wear out gradually. 

2. The tyres of vehicles wear out gradually due to friction.

The rubbing of tyres with road keeps on breaking tiny pieces of rubber from the tyre's surface gradually. Ultimately, all the treads present on the surface of a tyre are worn out and the tyre becomes baldy. Such baldy tyres must be replaced by new tyres. 

3. Friction wears out the rubbing machine parts.

Due to friction, the rubbing parts of a machine like ball bearings of bicycles wear out gradually.

4. Friction wear out the brake pads of vehicles gradually.

When the brakes of a vehicle are applied a lot of friction is produced between the brake pads and moving part of the wheel. This friction wears out the brake pads gradually.

5. Friction wears out steps of staircases in buildings and foot over bridges.

When a lot of people use the staircase in a building everyday, the friction between soles of their shoes and the stone steps wear away hard stone steps very very slowly.

6. Friction produces heat which may damage machines.

When the moving parts of of a machine rub together, a lot of heat is produced due to friction between them. This heat may damage the machine gradually. 

7. Friction reduces the efficiency of machines.

Due to friction a lot of energy is wasted as heat. This wastage of energy reduces the efficiency of a machine.

8. Friction slows down motion. 

Friction reduces the motion of moving parts of a machine. Infact, all the moving things are slowed down by friction.

METHODS OF INCREASING FRICTION:

1. Grooves are made in the soles of shoes to increase friction and prevent slipping.

2. Treads are made in the tyres of vehicles to increase friction and prevent skidding of vehicles on wet roads.

3. Spikes are provided in the shoes of players and athletes to increase friction and prevent slipping.

4. Gymnasts apply some coarse substance on their hands to increase friction for better grip.

5. Machine belts are made of special materials to increase friction and drive machine wheels properly. 

METHODS OF REDUCING FRICTION:

1. Friction can be reduced by making the surfaces smooth by polishing.

2. Friction can be reduced by applying lubricants to the rubbing surfaces.

The substances which reduces friction are called lubricants. When a few drops of oil are poured on the Hinges of a door the friction is reduced and the door moves smoothly. Friction can be reduced but friction can never be entirely eliminated.

3. Friction can be reduced by using wheels to move objects.

Friction can be reduced by attaching wheels or rollers to a heavy suitcase or any other heavy object which is to be moved. 

4. Friction can be reduced by using ball bearings between the moving parts of machines.

Ball bearing is a device which consists of a ring of small metal balls. Ball bearings are designed to make the moving parts of a machine to roll over each other rather than slide. Ball bearing makes the wheel roll smoothly over the axle. In most of the machines, friction is reduced by using ball bearings. 

FLUID FRICTION: FRICTION EXERTED BY LIQUIDS AND GASES:

Liquids and gases are called fluids. Water and air are the most common fluids. There is friction whenever an object moves through a fluid. It is called fluid friction. 

Air exerts frictional force on cars, buses, aeroplanes, rockets and birds etc. moving through it. Water exerts frictional force on objects like boats, speed boats, ships, submarines and fish etc. which move through it. 

The frictional force exerted by a fluid is called drag or drag force. 

The magnitude of frictional force or drag exerted by a fluid on an object moving through it depends on four factors. 

1. Speed of the object

2. Shape of the object

3. Size of the object

4. Nature of the fluid.

• Higher the speed of an object moving through a fluid, greater will be the frictional force or drag.

• The objects having streamlined shapes face much less frictional force or drag when moving through a fluid then the objects which do not have Streamline shapes.

• Larger the size of an object moving through a fluid greater will be the frictional force or drag acting on it. 

• Higher the viscosity or thickness of fluid greater will be the frictional force or drug acting on an object moving through it.

Streamline Shape: 

A streamlined body is a shape that lowers the friction drag between a fluid, like air and water, and an object moving through that fluid. It offers a minimum resistance to air and water by its particular type of body shape .

The aeroplanes and ships have streamlined shaped bodies to reduce drag. The streamlined shaped body experiences minimum resistance when travelling through water or air. It helps to reduce friction. Fishes, birds and rockets also have streamline shapes. 

DISADVANTAGES OF FLUID FRICTION

The main disadvantages of fluid friction are as follows. 

1. Fluid friction reduces the speed of objects moving through the fluids. 

2. When objects move through fluids they lost some of their energy in overcoming the fluid friction. This decreases their efficiency.

METHOD OF REDUCING FLUID FRICTION

The fluid friction can be reduced or minimised by giving special shape called streamlined shape to the objects which move through the fluids. 

Cars are built with streamlined body shape to reduce air resistance caused by air. 

An aeroplane has a streamlined shape to reduce air friction that it encounters when trying at high speed through the sky. 

MCQ -1: CLASS XII: PHYSICS ELECTROSTATIC CHARGE AND FIELDS

MCQ:

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

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20. 

 

21. 

 

22.

 

23. 

 

24. 

 

25. 

 

26. 

 

27. 

 

28. A body gets positive charge. It means that

a. It has lost electrons

b. It has gained protons

c. It has gained positrons

d. It has gained Alpha particles.

29. The minimum value of charge on any charged body may be

a. 1.6 x 10⁹ coulomb

b. 1 coulomb

c. 1 μC

d. 4.8 x 10⁻¹² C

30. A hemisphere is uniformly charged positively. The electric field at a point on a diameter away from the centre is directed

a. Perpendicular to the diameter

b. Parallel to the diameter

c. At an angle tilted towards diameter

d. At an angle tilted away from the diameter

LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 10 : REACHING THE AGE OF ADOLESCENCE

CLASS VIII   |    SCIENCE    |    CHAPTER 10

      notes prepared by subhankar Karmakar

                                                                         

Reaching the age of adolescence:

21. ADOLESCENT PREGNANCY:

In India, the legal age for the marriage of girls has been fixed at 18 years. Motherhood before 18 is known as adolescent pregnancy. 

Some of the problems brought about by early marriage in girls leading to early motherhood are given below:

a. The girls younger than 18 years of age are not prepared physically and mentally for motherhood.

b. Early marriage and motherhood cause health problems in the mother and of the child. 

c. Early marriage and motherhood cause I go to the girl as she is not prepared to fulfill the responsibilities of motherhood involved in bringing up the baby. 

d. Early marriage and motherhood katil the chances of higher education for girls. 

e. Early marriage and motherhood cartoons the employment opportunity for the young girls. 

22. REPRODUCTIVE HEALTH:

The reproductive health is defined as a state of physical, mental and social well being of a person in all matters relating to the reproductive system at all stages of life.

Conditions to maintain good reproductive health during adolescence are given below:

a. It is necessary to eat balanced diet during adolescence.

b. It is necessary to maintain personal hygiene during adolescence.

c. It is necessary to take adequate physical exercise during adolescence.

d. It is necessary to avoid taking any drugs during adolescence. 

23. NUTRITIONAL NEEDS OF ADOLESCENCTS

The diet which contains the correct amount of each constituent such as carbohydrates, fats, proteins, vitamins and minerals sufficient for the the normal growth and development of the body, and keep a person healthy, is called a balanced diet. Iron is a mineral which is necessary for making blood in the body. The iron rich foods such as leafy vegetables, Indian gooseberry, meet and jaggery are good for adolescencts. 

24. PERSONAL HYGIENE FOR ADOLESCENCTS

The maintenance of personal hygiene is necessary for adolescents for preventing diseases and keeping good health. They should take bath regularly and girls should take special care of cleanliness of the body during the time of menstrual flow.

25. PHYSICAL EXERCISE FOR ADOLESCENTS

All the adolescent boys and girls should do physical exercise such as brisk walking, jogging, swimming, cycling, dancing, playing outdoor games regularly. 

26. NO DRUGS FOR ADOLESCENCTS

Drugs are chemical substances which when taken into the body change the functions of the body, influence the mind and sometimes even change the behaviour of the person. 

Some of the harmful effects of taking drugs are as follows:

a. Drugs are addictive. If somebody take drugs once, he or she feel like taking them again and again and making them drug addicts. 

b. Drug addicts become irritable and lose interest in their studies or jobs.

c. Drugs do physical harm to the body. It damages brain, liver, lungs and kidney.

d. The sharing of syringes for injecting drugs spreads AIDS disease among drug addicts. AIDS is a dangerous disease which is caused by a virus called HIV. 

AIDS = Acquired Immune Deficiency Syndrome

HIV = Human Immunodeficiency Virus

AIDS can be spread in the following ways

a. AIDS virus can pass from an infected person to a healthy person by the sharing of syringes used for injecting drugs.

b. It can be transmitted to a healthy person through sexual contact with a person infected with HIV.

c. It can be transmitted to an infant from the infected mother through her milk. 

27. HORMONES OTHER THAN SEX HORMONES

Some of the important hormones other than sex hormones are : Growth hormones, thyroxine, insulin and adrenaline. 

A. Pituitary gland

Pituitary is an endocrine gland. It is attached to the base of the brain. It secrets in number of hormones. One of the hormone secreted by pituitary gland is the growth hormone. The growth hormone controls the growth of the human body. A person having a deficiency of growth hormone in childhood remains very short and becomes a dwarf. A person having too much growth hormone becomes very tall. 

Pituitary gland is called master gland, because many of the hormones which its secrets control the functioning of other endocrine glands in the body like testes, ovaries, thyroid gland and adrenal glands. 

B. Thyroid gland

Thyroid is a large endocrine gland in the neck. Thyroid gland makes a hormone called thyroxine which contains iodine. A deficiency of iodine in the diet can cause a deficiency of thyroxine hormone in the body. The deficiency of thyroxine hormone causes a disease known as goitre. Main symptoms of goitre disease is that the neck of the person suffering from goitre appears to be swollen. People are advised to use iodised salt for cooking food so as to prevent Goitre disease. 

C. Pancreas

Pancreas just below the stomach in our body. Pancreas secrete the hormone called insulin. The function of insulin hormone is to lower the blood sugar level for blood glucose level. Deficiency of insulin hormone in the body causes a disease known as diabetes. 

D. Adrenal glands or adrenal

Adrenal endocrine glands there are two adrenal glands in our body which are located on the top of two kidneys. The adrenal glands produce adrenaline hormone. The adrenaline hormone prepares our body to function at maximum efficiency during emergency situations like danger, fear, shock, surprise or excitement. 

When we are faced with a dangerous situation then the adrenal glands secrete more adrenaline hormone into our blood. This adrenaline hormone speeds up the heartbeat, increases breathing rate, raises blood pressure, and causes liver to release more stored glucose into blood. All these actions of adrenaline hormone produce a lot of energy in our body very very quickly. 

The adrenal glands also produce and secrete another hormone called aldosterone. The aldosterone hormone secreted by adrenal glands maintains the correct salt balance in the blood. 

ROLE OF HORMONES IN COMPLETING THE LIFE HISTORY OF FROG AND INSECT

LIFE OF FROG

In a frog, the tadpole or Larva hatched from the eggs passes through certain stages to become a frog. The change of tadpole to the adult frog is called metamorphosis. In a frog, metamorphosis is brought about by thyroxine hormone, produced by the thyroid gland. The production of thyroxine hormone requires the presence of iodine in water. So is the pond water in which the tadpoles are growing does not contain sufficient iodine to make enough thyroxine hormone there will be deficiency of thyroxine hormone due to which tadpoles cannot undergo metamorphosis and hence cannot become adult frogs. 

All the amphibians need thyroxine hormone to undergo metamorphosis and change from larvae into adults. 

LIFE OF SILK MOTH

The caterpillar or larvae has to pass through to various stages and undergo metamorphosis to become an adult silk moth. Silk moth is a kind of insect the process of metamorphosis in insects is controlled by insect hormones.  

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 11 : FORCE AND PRESSURE

FORCE

 

·         A push or pull on an object is called force.   

·         Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing.

 

Force is due to an interaction:

 

·         When two objects interact with each other, they can exert forces on each other. These forces can either attract or repel the objects, depending on the nature of the interaction.

 

Force has magnitude as well as direction:

 

·         The strength of a force is expressed by its magnitude. 

·         The magnitude of force is expressed in the the SI unit of force called Newton. Its symbol is N.

·         1 Newton is the force which can make an object of 1 kg mass to move an acceleration of 1 m/s².

·         When two forces act along the same direction, then the net force acting on the object is equal to the sum of the two forces and when they act in opposite direction, and then the net force acting on the object is equal to the difference between the two forces. 

·         If the two forces applied to an object are equal in magnitude and act in opposite directions, then the net force acting on the object is zero.

 

EFFECTS OF FORCE:

 

A force can produce the following effects.

·         A force can move a stationary object.

·         A force can stop a moving object.

·         A force can change the speed of a moving object. 

·         A force can change the direction of a moving object. 

·         A force can change the shape and size of an object.

 

TYPES OF FORCES

The most common types forces are:

·         Muscular force

·         Frictional force

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

CONTACT FORCES

A force which can be exerted bi an object on another object only through physical touching or contact is called a contact force. Examples of contact forces are

·         Muscular force and

·         Frictional force

 

MUSCULAR FORCE

The force exerted by the muscles of the body is called muscular force.

As the muscular force can be applied to an object only when our body or body of an animal is in contact with the object, therefore, muscular force is a contact force.

 

FRICTIONAL FORCE OR FRICTION

The force which always opposes the motion of one body over another body is called frictional force or friction. 

Since frictional force arises only when the surfaces of two objects are in touch with each other, frictional force is an example of of contact force. 

NON CONTACT FORCES

A force which can be exerted bi an object on another object even from a distance without touching each other is called a non contact force.

The examples of non contact forces are

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

MAGNETIC FORCE

The force exerted by a magnet is called magnetic force. 

Since a magnet can exert its magnetic force on iron objects from a distance even without touching them, therefore, magnetic force is a non contact force. The magnetic force between a magnet and an iron object is always that of attraction. A magnet can attract or repel another magnet. 

 

A magnet has two poles, North Pole and South Pole. 

 

There is a magnetic force of repulsion between the like poles of two magnets and there is a magnetic force of attraction between the unlike poles of two magnets. 

 

The magnetic force is widely used in our everyday life. 

 

ELECTROSTATIC FORCE

There are two types of electric charges. They are Positive electric charges and Negative electric charges. 

 

The force exerted by an electrically charged object is called electrostatic force. As an example, when a plastic comb is rubbed in dry hair, it gets electric charges and this electrically charged comb can attract tiny pieces of paper. 

 

Electrostatic forces may be attractive or repulsive. Electrostatic force between two like charges is repulsive and between two unlike charges is attractive. Therefore we can say, a positive charge repel another positive charge but attracts a negative charge. 

 

The electrostatic force can be exerted by a charged object on another object from a distance even when they are not in touch with each other; therefore, electrostatic force is an example of non contact forces. 

 

GRAVITATIONAL FORCE

The pull exerted by objects possessing mass on another mass is called gravitational forces. Gravitational force between two objects is a force of attraction. 

 

It is the gravitational force between the sun and the earth which holds the earth in its orbit around the sun. 

The force with which the earth is the objects towards it, is called the force of gravity or simply gravity. 

The force of gravity causes all the objects to fall towards the earth. 

The gravitational force of Earth or gravity acts on objects from a distance without there being a physical contact, therefore, gravitational force or gravity is an example of non contact forces. 

PRESSURE

 

Pressure is produced when a force acts on an object. The force acting on an object per unit surface area of the object is called pressure. 

The effect of a force depends on the area of the object on which it acts. 

Pressure = Force / Area

The SI unit of pressure is Newton per square metre ( N/m²), which is also called Pascal (Pa).

1 Pa = 1 N/m² 

1 kilo Pascal (1 kPa = 1000 Pa) is equal to 1000 Pascal. 

FACTORS ON WHICH PRESSURE DEPENDS

The pressure depends on two factors.

·         The force applied

·         Area over which force acts. 

The same force can produce different pressures depending on the area over which it acts. For example, when a force acts over a large area of an object, it produces small pressure. But if the same force acts over a small area of the object, it produces a large pressure. 

 

WHY SCHOOL BAGS HAVE WIDE STRAPS

A school bag or a shoulder bag has wide strap made of thick cloth so that the weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure it is more comfortable to carry the heavy school bag. 

 

WHY A SHARP KNIFE CUTS BETTER THAN A BLUNT KNIFE

A sharp knife has a very thin edge to its blade. A sharp knife cuts objects like vegetables better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hands falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty. 

 

WHY THE TIP OF A NEEDLE IS SHARP

The tip of a sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth producing a large pressure sufficient to Pierce the cloth being stitched. A knife Razer blade and an x are the cutting tools where is a sewing needle is a piercing tool. 

 

WHY THE DEPRESSION IS MUCH MORE WHEN A MAN STANDS ON THE CUSHION THEN WHEN HE LIES DOWN ON IT

When a man stands on a cushion then only his two feet having small area are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion then his whole body having larger area is in contact with the cushion producing much smaller pressure. This smaller pressure produces a very little depression in the cushion. 

 

PRESSURE EXERTED BY LIQUIDS

All the liquids exert pressure on the base and the walls of their containers. All the liquids have weight and this weight acts on the base of the vessel producing a pressure. 

The pressure exerted by a liquid increases with increasing depth inside the liquid. 

The pressure of a liquid increases with depth. The greater the depth of a point in a liquid the greater is the pressure. Liquids also exert pressure on the walls of the vessel in which they are stored. The sideways pressure exerted by liquids also increases with the depth of the liquid. 

The formation of mountains of water from the leaking pipes of water supply pipeline tells us that water exerts pressure on the walls of the container. 

A liquid exerts pressure in all directions even upwards. 

 

PRESSURE EXERTED BY GASES

 

All the gases exert pressure on the walls of their containers. Air pressure arises due to the constant solutions of the tiny molecules of the gases present in air with the walls of the container or vessel in which it is enclosed. 

High air pressure produces by the gas molecules on the walls of balloon causes it to extend and get inflated. 

 

Two examples that gases like air exert pressure are:

 

·         When air is filled into a balloon with our mouth the balloon gets inflated.

·         When air is filled into a bicycle tube with a pump the tube gets integrated and makes the tyre feel hard. 

 

ATMOSPHERIC PRESSURE

The layer of air above the earth is called atmosphere. Air has weight show the atmosphere consisting of tremendous amount of air has enormous weight the weight of atmosphere exerts a pressure on the surface of the earth and on all the objects on the Earth including ourselves. This pressure is known as atmospheric pressure. The atmospheric pressure is due to the weight of air present in the atmosphere above us. Atmospheric pressure also acts in all directions. 

 

MAGNITUDE OF ATMOSPHERIC PRESSURE

 

Atmospheric pressure is equal to the weight of air present in a BD tall column of air standing on one square metre area of the earth.  As we go up in the atmosphere from the surface of Earth the atmospheric pressure goes on decreasing. On the surface of Earth, the atmospheric pressure is maximum at the sea level. The atmospheric pressure on the top of a high mountain will be much less than at its base. 

Although pressure is is measured in Pascal, atmospheric pressure is measured in mm of Mercury. The atmospheric pressure on the surface of earth at the sea level is 760 mm of Mercury. 

 

OUR BODY AND ATMOSPHERIC PRESSURE

Change the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside we do not get crushed. But there is an effect of low atmospheric pressure in our body. At higher altitudes, atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes. 

 

APPLICATIONS OF ATMOSPHERIC PRESSURE IN EVERYDAY LIFE

 

1. DRINKING STRAW

 

The drinking straw works on the existence of atmospheric pressure. The lower end of drinking straw is dipped in the soft drinks. When we suck at the upper end of the straw with our mouth, the pressure of air inside the straw and in our mouth is reduced. The pressure acting on the surface of the soft drink is equal to atmospheric pressure. So, the greater atmospheric pressure acting on the surface of the soft drink pushes the soft drink up the straw into our mouth. 

 

2. SYRINGE

 

A glass tube or plastic tube with a nozzle and piston for sucking in and ejecting liquid in a thin stream is called a syringe. The series works on the existence of atmospheric pressure. When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe. 

3. DROPPER

 

The dropper is a small glass tube with a rubber ball at one end and a nozzle at the other. Its functionality relies on atmospheric pressure. To use the dropper, we immerse the nozzle in the liquid and squeeze the rubber ball, causing air inside the tube and bulb to escape as bubbles. This action lowers the air pressure within the dropper. Once we release the rubber bulb, the higher atmospheric pressure outside the dropper forces the liquid to be drawn up into the tube.

 

4. RUBBER SUCKER

 

A rubber sucker, also known as a suction cup, is a rubber device that adheres firmly to flat and smooth surfaces when pressed. Its appearance resembles a small concave rubber cup. When we apply the rubber sucker to a flat, smooth surface and press it, the concave rubber cup flattens significantly, expelling most of the air from underneath. This compression results in a significant reduction in air pressure inside the rubber sucker. As a consequence, the higher atmospheric pressure from the outside effectively secures the rubber sucker in place on the flat surface.