ICSE PHYSICS||Sound|| Chapter-7B||Natural And Damped Vibrations||Forced Vibrations||Resonance||

UNIT A NUMERICALS:

Sound is the form of energy which is produced by vibrating bodies and which causes sensation of hearing in us.
A mechanical wave is a wave that is not capable of transmitting it's energy through vacuum. They require a medium in order to transport their energy from one location to another.
we can't hear the sounds produced by all vibrating bodies. We only can hear the sound with frequency range 20Hz to 20kHz

WAVE :The disturbance produced in the medium which can travel in all directions with same speed is called Wave. During the Propagation of wave particles of the medium don't m9ve rather they will vibrate from their mean position.

TERMS RELATED TO THE WAVE :
(i) Amplitude (a) — When sound waves travel in a medium, the maximum displacement of the particle of medium on either side of its mean position, is called amplitude of the wave.

(ii) Frequency (f) — The number of vibrations made by the particle of the medium in one second.

(iii) Wavelength (λ) — The distance travelled by a wave in one time period of vibration of the particle of the medium, is called the wavelength.

(iv) Wave velocity (V) — The distance travelled by the wave in one second.

The two factors on which the speed of a wave travelling in a medium depends are —
(i) Elasticity and (ii) Density

ECHO-CONDITIONS:
The sound heard after reflection from a distant obstacle (such as a cliff, a hillside, the wall of a building, edge of a forest, etc.) after the original sound has ceased, is called an echo.

The two conditions for an echo to be heard distinctly are —

(1)The minimum distance between the source of sound (or observer) and the reflector in the air must be 17 m.(2)The size of the reflector must be large enough as compared to the wavelength of the sound wave

APPLICATION OF AN ECHO:

Dolphins detect their enemy and obstacle by emitting the ultrasonic waves and hearing their echo.

In medical field, echo method of ultrasonic wave is used for imaging the human organs (such as liver, gall bladder, uterus, womb, etc.) This is called ultrasonography.

NATURAL VIBRATIONS: The periodic vibrations of a body in the absence of any external force on it, are called the natural (or free) vibrations.
For example —
A body clamped at one point when disturbed slightly from its rest position, starts vibrating. The vibrations so produced are called the natural or free vibrations of the body.
The natural frequency is the frequency at which a body tends to oscillate in the absence of any driving or damping force.

The natural frequency of vibrations of a body depends on the shape and size (or structure) of the body.

DAMPED VIBRATIONS: The periodic vibrations of a body of decreasing amplitude in presence of a resistive force are called the damped vibration.

In the case of damped vibrations, the amplitude of vibrations gradually decreases with time and ultimately it ceases whereas in the case of free vibrations the amplitude of vibrations remains constant and it continues forever.
Example of damped vibration — A tuning fork when stroked on a rubber pad, executes the damped vibrations in air.
FORCED VIBRATIONS:
The vibrations of a body which take place under the influence of an external periodic force acting on it, are called forced vibrations.

Example — The vibrations produced in the diaphragm of a microphone sound box with frequencies corresponding to the speech of the speaker, are the forced vibrations.

RESONANCE:
Resonance is a special case of forced vibrations.

When the frequency of the externally applied periodic force on a body is equal to its natural frequency, the body readily begins to vibrate with an increased amplitude. This phenomenon is known as resonance.
At resonance, the body vibrates with a large amplitude thus conveying more energy to the ears, so a loud sound is heard.
APPLICATIONS:
(1)When a troop crosses a suspension bridge, the soldiers are asked to break their step, because when the soldiers march in steps, each soldier exerts a periodic force in same phase and therefore the bridge executes the forced vibrations of frequency equal to the frequency of their steps.

Now, if the frequency of the steps becomes equal to the natural frequency of the bridge, the bridge will vibrate with large amplitude due to resonance and hence, the suspension bridge may collapse.

(2)The stringed instruments like guitar are provided with a hollow soundbox containing air because when strings are made to vibrate, forced vibrations are produced in air of the sound box.

The surface area of air in the sound box is large, so the forced vibrations of air send forth a greater energy and cause a loud sound.

[praveenwilson]

Some one give this a man a noble prize in physics

[GRIMREAPER_]

The notes i write while listening to your teaching is more than anything ive written in school and i wrote little to nothing.
Thank you for teaching such topics so cheerfully and in a method that is easy for us studends to understand..

[weeeee]

Thank you so much sir for listening to our requests. The videos are very much helpful 😊🙏

[xavierpjose]

Explanations are crystal clear and authentic.

[anonymousis]

Sir small error at 21:53
If you take
2L : L : 2L/3
Then lcm is three and you will get ratio as 6:3:2

2L/2 had to be written as L only I think
But thank you sir, video helped a lot🙏

[swapnap]

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[adyaprasad]

Thank you sir, your videos are very helpful.

[rvr.v]

Thank you sir
Explanations are very clear.

[Sritama-fnto]

Really this helped me a lot ❤️
And the 'dekha to ye jana sanam' humming part really made me listening your video with more energy 😊❤️
Thank you so much Sir 🙌🙏

[sumairaakram]

Sir i usually don't comment but your video forced me ...
Thanks you sir ❤

[sweetfruit]

Hello sir pls pls pls im requesting to u pls upload 7(c) part of explanation of sound chp pls sir bcz my exam of physics is on 12 September
Pls sir help me

[dishuachint]

this video helped me a lot for my physics exam which is tomorrow, very helpful 🤩🤩. sir wanted to ask do u teach maths also ?

[shilpamas]

sir give one shot video sir, we will use it for exam

[beereshsk]

Sir for icse 10th board exam which study material can be helpful, and even which question blank will be useful sir.😊pls tell sir 🙏🏻....

[Hero-]

18:38 Sir how is it lamda by 2 sir ? How did this expression come sir ? Could you please explain ??

[SceincXperiment]

sir, Why amplitude increases in forced vibration(resonance) because in resonance Air friction will be there