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.