Class 9 Science

Sound

In Text Question Answer

Question 1: How does the sound produced by a vibrating object in a medium reach your ear?

Answer: Vibrations in an object create disturbance in the medium and consequently compressions and rarefactions. Because of these compressions and rarefactions sound reaches to our ear.

Question 2: Explain how sound is produced by your school bell.

Answer: School bell starts vibrating when heated which creates compression and rarefaction in air and sound is produced.

Question 3: Why are sound waves called mechanical waves?

Answer: Since sound waves do some mechanical work while making disturbance in medium, hence sound waves are called mechanical wave.

Question 4: Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer: Since sound waves require medium to propagate and there is no medium present on the moon. So, I will not able to hear the sound of my friend on the moon.

Question 5: Which wave property determines (a) loudness, (b) pitch?

Answer: (a) Amplitude of sound waves determines loudness. Louder sound has greater amplitude and vice versa.

(b) Frequency of the sound waves determined pitch of the sound.

Question 6: Guess which sound has a higher pitch: guitar or car horn?

Answer: Sound of the car horn has higher pitch.

Question 7: What are wavelength, frequency, time period and amplitude of a sound wave?

Answer: Wavelength: Wavelength is the distance between two consecutive compressions or rarefaction of wave.

Frequency: The number of sound wave produced in one second is called frequency.

Time period: Time period is the time taken to produce one wave of sound.

Amplitude: Amplitude is the maximum displacement along the mean position of the particles of medium.

Question 8: How are the wavelength and frequency of a sound wave related to its speed?

Answer: The relation between frequency and wavelength of sound wave is given as follows:

`text(Velocity)=text(Wavelength)xx\text(Frequency)`, `v = λxx ν`

This means the speed is equal to the product of wavelength and frequency of the sound wave.

This equation is also called the 'wave equation' and applicable to all types of wave.

Question 9: Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Answer: Given, Frequency (ν) = 220 Hz
Velocity (v) = 440m/s
Wavelength (λ) =?

We know;

`v=λxxν`
Or, `440m//s=λxx220Hz`
Or, `λ=(440ms^-1)/(220Hz)=2m`

Thus, wavelenght = 2m

Question 10: A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Answer: Since, the time interval between successive compressions is called time period or time interval.

Here given, Frequency (ν) = 500Hz

T (Time period) =?

We know that;

Frequency`=1/T`

Or, `T=1/text(Frequency)`

Or, `T=1/(500Hz)=0.002s`

Thus, time interval between two consecutive compression of the given wave = 0.2 s

Question 11: Distinguish between loudness and intensity of sound.

Answer: Loudness of sound is determined of amplitude and intensity of the sound wave is determined by frequency of sound waves.

Question 12: In which of the three media; air, water or iron does the sound travel the fastest at a particular temperature?

Answer: At particular temperature sound travels fastest in iron.

Question 13: An echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m /s?

Answer: To return an echo sound has to cover distance of two way.

Here, given, Speed of sound = 342 m/s

Time = 3s

Thus, Distance = speed X time

⇒ Distance `= 342m//sxx3 s=1026 m`

Thus,the distance between the source and reflecting surface`=1026÷2=513m`

Question 14: Why are the ceilings of concert halls curved?

Answer: Since, concert halls are big, so audience at the back rows of the hall may not hear clear sound of speaker. To overcome this problem, the ceiling of the concert halls is made concave. Concave ceiling helps the sound wave to reflect and send to farther distance which makes the concert hall enable to send clear sound to the audience even sitting in back rows of hall.

Question 15: What is the audible range of the average human ear?

Answer: 20 Hz to 20000 Hz

Question 16: What is the range of frequencies associated with?

(a) Infrasound
(b) Ultrasound

Answer: (a) Infrasound: Less than 20 Hz
(b) Ultrasound: More than 20000 Hz

Question 17: A submarine emits a SONAR pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?

Answer:To return the SONAR pulse back, its wave has to travel two way.

Here, given, Velocity (v) of sound wave = 1531m/s
Time (T) = 1.02 s

Thus, Distance = speed X time

Distance`=1531 ms^-1\xx1.02 s=1561.62 m`

So,the distance between the source and reflecting surface`=1561.62÷2=780.81m`


Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.

Tissue

A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.

Motion

If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.

Gravitation

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.

Sound

Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.