grade 8 science worksheet,8th grade physical science worksheets pdf,class 8 science worksheets
grade 8 science worksheet,8th grade physical science worksheets pdf,class 8 science worksheets
grade 8 science worksheet,8th grade physical science worksheets pdf,class 8 science worksheets
Sound
When we tune into radio, when we turn on the television, we hear sound . if we strike a drum with drumstick, it vibrates. Every objects vibrates while they produce sound. Hence, sound is the form of energy that is produced by the vibration of an object. Drum, madal, tuning fork flute etc are the sources of sound. We hear both pleasant and unpleasant sounds. The sounds that makes our ear feels beautiful like the sound of birds chirping, flowing water in the river, music are pleasant sounds. The sounds that make our ears irritate and disturbing are unpleasant sound, they are also called as noise. Like, sounds of automobiles, heavy machines, industries etc.
Loudness of sound:
If we tune into radio and make low volume we hear soft sound, when we make the volume high we hear louder sound. If we touch the speaker, we can feel the speaker vibrating more.
Hence, loudness of sound increases with the increase in vibration of the body.
When we pluck the string of guitar slowly, a soft sound is heard and the vibration will also be low. When we plucked it hard, loud sound is heard and vibration will also be more.
Sharpness of sound:
Sharpness of sound depends on the speed of vibration. If an object vibrates very fast, sharp sound is produced. Also, sharpness of sound depends on size of an object. If the size of an object is smaller, sharp sound is produces.
Can a body produce both sharp and dull sound?
Yes, a body can produce both sharp and dull sound. If it produces dull sound in normal condition, it can produce sharp sound when we stretch it.
Boys produce dull sound and girls produce sharp sound because boys have large sized vocal cords and girls have small one.
Wave:
Take a small piece of stone and throw into the pond, it produces ripples. Those ripples are called waves. Waves are produced due to disturbance in the medium.
Waves are of two types:
Transverse waves
Longitudinal waves
Sound waves are longitudinal waves. They move from one point to another by making compression and rarefaction.
Propagation of sound:
Propagation of sound is defined as the process of transferring of the vibrational energy from one point to another in the form of sound waves.
Sound doesn’t travel in a vacuum, sound needs material medium to travel from one place to another.
Propagation of sound through solids:
Sounds needs medium to travel, the molecules in the solids are highly compressed so less vibrational energy is used to transfer sound from one molecule to another.
Thus, the speed of sound in solid in the highest.
Propagation of sound in liquid:
Liquid can also propagate the sound. The molecules of the liquids have less intermolecular distances than that of gases. So travelling of sound in liquid is faster that the gases but slower than that of solids.
Propagation of sound in gases:
We speak something with our friends and families. They will listen to us because our sound waves travel in a medium and the air is the gaseous medium. But the speed of propagation of sound in gaseous medium is less than that of in solids and liquids.
Speed of sound in different medium:
Factors affecting speed of sound in gases:
Density:
The speed of sound decreases when density of the gas increases. Carbon dioxide is heavier than nitrogen, so speed of sound is less in carbon dioxide than in nitrogen.
Temperature:
When temperature increases, the speed of sound also increases.
Humidity:
Amount of water vapour present in air is called humidity. Speed of sound increases when humidity of air increases.
Direction of motion of air:
The speed of sound is more in direction of air. Suppose, air is moving from east to west, the sound travels faster towards the west direction than in the east direction.
Wavelength:
The distance between two compressions or two rarefactions of the sound wave is called wavelength. It is denoted by lamda (λ ).
grade 8 science worksheet,8th grade physical science worksheets pdf,class 8 science worksheets
Amplitude(a)
The maximum displacement of a vibrating particle from its mean position is called its amplitude. Its SI unit is metre. If the amplitude is higher, louder will be the sound.
Frequency (f)
The number of complete vibrations which are produced in per second is called its frequency. Frequency is measured in Hertz (symbol Hz). If a body makes 30 complete vibrations in one second, we can say that its frequency is 30Hz.
Time period(T)
The time taken by a vibrating body to complete one vibration is called its time period. The time period is measured in seconds, if a body takes 0.5s to complete one vibration,we can say that its time period is 0.5s.
Time period = 1/Frequency = 1/f
Velocity (V)
The distance travelled by a sound in per second is called its velocity.
Its SI unit is m/s, ie
T = (Distance travelled by sound (s))/(Time taken (t) )
Types of sound waves:
Infrasonic sound
The sound wave having the frequency less than 20Hz is called infrasonic sound. It is produced due to vibration of very large objects. It cannot be detected by human air.
Example, sound produced during earthquakes.
Audible sound
The sound wave having the frequency between 20Hz and 20 KHz is called audible sound. It is produced due to vibration of pipes, strings, drums, human, vocal cord etc.
Ultrasonic sound:
The sound wave having the frequency greater than 20KHz is called ultrasonic sound. It is produced by vibration of very small objects. This sound also cannot be detected by human ear. Animals like dog, bats, dolphins etc can hear this sound.
Reflection of Sound:
The phenomenon of the turning back of sound to the previous medium after striking any obstacle on its way of propagation is called reflection of sound.
Echo:
An echo is the repetition of sound cause due to reflection of the sound by an obstacle at a distance at least 17m away from the source.
Reverberation:
The repeated reflection of sound by an obstacle at the distance less than 17m that results in prolongation of sound is called reverberation.
Notes:
Sound cannot travel through a vacuum.
Astronauts can use electric medium to talk with each other on the moon. Why?
When a ringing bell is touched, sound is not heard because the vibration is absorbed by our hand.
Sound is repeated near hills due to formation of the echo as the reflection of the sound by nearby hills takes place.
An echo is not heard in a small room because the source and obstacle must be at least 17m.
EXERCISES:
Answer all of the following questions:
Define sound and write different sources of sound.
What do you mean by pleasant and unpleasant sound? Write with examples.
Why does sound travels faster in solid state than liquid and gas?
Can a body produce both sharp and dull sound at once?
What do you mean by propagation of sound? Can sound travel in vacuum?
Show an activity to demonstrate that sound travels through solid.
Write an experiment to show sound travels in liquid.
How is sound produced? What do you mean by loudness of sound?
Why the sound of a girl is softer than
What are the factors that affect speed of sound in gases? Explain how they affect the speed of sound.
Define the terms wavelength, amplitude and frequency.
What are different types of sound waves? Write with examples.
What do you mean by echo and reverberation?
Why the ceilings on concert halls, cinema halls, and conference halls are curved?
Give reason why astronauts use electronic medium to talk on the moon.
We hear a louder sound in a newly constructed room than a furnished one. Why?
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