GSEB Solutions for Class 9 Science and Technology – Wave Motion and Sound

GSEB Solutions for Class 9 Science and Technology – Wave Motion and Sound (English Medium)

GSEB SolutionsMathsScience
Exercise 22:

Solution 1.1:

D. 4A
When an oscillator starts its motion from the equilibrium position and reaches the maximum displacement (A) on any side of the equilibrium position, returns back and reaches the maximum displacement on the other side of the equilibrium position via the equilibrium point and again returns to the equilibrium position, then it is said to have completed one oscillation.

Solution 1.2:

C. It can be either longitudinal or transverse.
Sound waves require a material medium for their transmission; hence these are mechanical waves which can either be transverse or longitudinal. 

Solution 1.3:

C. It can be either longitudinal or transverse.
Sound waves propagating in solids can be either longitudinal or transverse.

Solution 1.4:

B. Light waves
Light waves being non-mechanical waves do not require a material medium for their propagation.

Solution 1.5:

A. Infrasonic
Earthquake waves have frequencies less than 20 Hz, thus they are infrasonic waves.

Solution 1.6:

B. Sound Navigation and Ranging
SONAR stands for Sound Navigation And Ranging.

Solution 1.7:

C. 0.017 m to 17 m

Exercise 23:

Solution 1.8:

D. 30000 Hz
Sound having a frequency of more than 20000 Hz is known as ultrasonic.

Solution 1.9:

A. If the time interval between original sound and reflected sound is more than 1/10 s.
If the time interval between the original sound and reflected sound is more than 1/10 s, then the original sound and reflected sound can be clearly heard. This reflected sound is known as an echo.

Solution 1.10:

Wave velocity = Wavelength x Frequency
Thus, we can say that wavelength is inversely proportional to the frequency.

Solution 2.1:

Yes, all objects in oscillatory motion also exhibit periodic motion.
Oscillatory motion which takes place along a certain path, about a fixed point at a certain time interval, is also periodic.

Solution 2.2:

No, all objects in periodic motion may not exhibit oscillatory motion since all periodic motions may not be to and fro.

Solution 2.3:

Periodic time = 1/Frequency

Solution 2.4:

Transverse waves propagate through crests and troughs.

Solution 2.5:

Longitudinal waves propagate through condensations and rarefactions.

Solution 2.6:

If the particles of a medium oscillate in the direction parallel to the direction of propagation of the wave, the wave is said to be a longitudinal wave.

Solution 2.7:

A light wave is a non-mechanical wave, i.e., it does not require a mechanical medium for its propagation. It can also travel in vacuum.

Solution 2.8:

The distance travelled by a wave in the direction of wave propagation during time equal to periodic time is known as the wavelength of the wave.

Solution 2.9:

The range of frequency of audible sound of the human ear is 20 Hz to 20000 Hz.

Solution 2.10:

For an echo to be heard distinctly, the minimum distance between the source of sound and reflecting surface should be 17 m.

Solution 2.11:

The unit of intensity of sound is, weber per metre square (W /m²).

Solution 3.1:

An object is said to be oscillating if it repeats its back and forth, to and fro or up and down motion, along a certain path, about a fixed point at regular time intervals.

Solution 3.2:

Examples of the objects performing periodic motion are motion of the pendulum of a clock, motion of a swing, motion of the Earth around the Sun etc.

Solution 3.3:

The maximum displacement of a simple harmonic oscillator, from the equilibrium position in any direction is known as the amplitude of the oscillator.

Solution 3.4:

The motion of a disturbance in a medium or space is known as a wave.
Wave is not a physical object but an effect. Due to the disturbance in a medium, particles of the medium oscillate and these oscillations produce a wave.

Solution 3.5:

Sound waves having frequency more than 20 kHz are known as ultrasonic sounds.
Animals like bats, dogs, cats, some birds and insects are sensitive to such sounds.

Solution 3.6:

The sound heard after reflection from a distant obstacle such as a cliff, hillside, wall of a building etc. after the original sound has ceased, is called an echo.
For an echo to be heard distinctly, the time interval between the original sound and reflected sound should be more than 1/10 s.

Solution 3.7:

The uses of ultrasonic sounds are:

1. They are used in the medical field for diagnosis and treatment of internal illnesses of the human body.
2. Using ultrasound, kidney stones can be broken into small grains which later on get flushed out through the urine.
3. Ultrasonography is used to examine the foetus during pregnancy.
4. Ultrasound is also used in heart checks (ECG).
5. Industrially, ultrasonic sounds are useful in examining metals for defects like cracks etc. and for cleaning machine parts.

Exercise 24:

Solution 3.8:

SONAR (Sound Navigation and Ranging): Sonar is a device fitted in sailing ships, trawlers, war ships, etc., to locate submarines or shoals of fish or to determine the depth of the ocean bed.
Principle: It is based on the principle of reflection of sound waves.
Working: Powerful pulses of ultrasonic sounds are sent out at regular intervals from a transmitter mounted on a ship. When these pulses are intercepted by submerged objects, they get reflected. The reflected sound or echo is detected by an underwater receiver, which is also mounted on the ship. If the speed of ultrasound is v and t is the elapsed time between the transmission and the reception of the reflected ultrasound signal, the depth of the submerged object underwater is:
h = vt/2
This method is called echo depth sounding.
Applications: 

1. To determine the depth of sea or ocean.
2. To detect an iceberg.
3. To detect enemy submarines during war times.

Solution 3.9:

Waves which require a material medium for their propagation are known as mechanical waves.
Example: Waves on a water surface, sound waves etc.

Solution 3.10:

Waves which do not require a material medium for propagation are known as non-mechanical waves. Such waves can even travel in a vacuum.
Example: Light waves, electromagnetic waves.

Solution 3.11:

Waves in which the particles of the medium oscillate in the direction parallel to the direction of propagation of wave are called longitudinal waves.
Example: Sound waves.

Solution 3.12:

Waves in which the particles of the medium oscillate in the direction perpendicular to the direction of propagation of wave are called transverse waves.
Example: Waves on a string.

Solution 3.13:

Loudness is the property of sound by virtue of which a loud sound can be distinguished from a faint one, i.e., it gives us the extent of effect of sound.
Loudness is subjective in nature while intensity, which can be measured is objective. Sounds of the same intensity might be very loud for one person and not very loud for another person. Intensity of sound is measured as the sound energy passing through a cross section of unit area in unit time.
The unit of intensity of sound is weber per metre squared (W/m²) while that of loudness is decibel (dB).

Solution 3.14:

When the distance between the source of sound and reflector is less than 17 m, due to repeated reflections, the original sound mixes with the reflected sound and the sound is prolonged. That is, sound is heard for sometime even after the production of sound stops. This phenomenon is called reverberation.

Solution 3.15:

1. . Periodic motion: If a body repeats its motion after a fixed time interval, along a certain path and about a fixed point, it is said to have periodic motion.
2. Simple harmonic motion: Oscillatory motion along a linear path executed by a body about a fixed point, under the action of a force proportional to its displacement from that point and with direction towards that point is called a simple harmonic motion.
3. Wave: The motion of a disturbance in a medium or space is known as a wave. A wave is not a physical object but an effect. Due to the disturbance in a medium, particles of the medium oscillate and these oscillations produce a wave.
4. Wavelength: The distance travelled by a wave in one time period of vibration of article of medium, is called its wavelength. It depends on the medium in which the wave travels. Its S.I. unit is metre (m).
5. Amplitude of a wave: During the propagation of a wave, the maximum displacement of the article of medium on either side of its mean position is called the amplitude of the wave. Its S.I. unit is metre (m).
6. Frequency of a wave: Number of waves passing through any point in the medium in one second is known as the frequency of the wave.
7. Wave velocity: Distance travelled by disturbance in unit time in the direction of propagation of the wave is known as the wave velocity.
   Mathematically, it is equal to the product of wavelength and its frequency. Its S.I. unit is metre per second.
8. Audible sound: Sound in the frequency range of 20 Hz to 20000 Hz can produce sensation on a healthy human ear and such sound is called audible sound.
9. Infrasonic sound: Sounds having frequency less than 20 Hz are known as infrasonic sounds. We cannot hear such sounds.
10. Ultrasonic sound: Sounds having frequency more than 20000 Hz are known as ultrasonic waves. Animals like bats, dogs, cats etc. can hear such sound.
11. Echo: The sound heard after reflection from a distant obstacle such as a cliff, hillside, wall of a building etc., after the original sound has ceased, is called an echo. For an echo to be heard distinctly, the time interval between the original sound and reflected sound should be more than 1/10 s.
12. Reverberation: When the distance between the source of sound and reflector is less than 17 m, due to repeated reflections, the original sound mixes with the reflected sound and the sound gets prolonged. That is, sound is heard for sometime even after the production of sound stops. This phenomenon is called reverberation.

Solution 4.1:

Solution 4.2:

Solution 4.3:

Solution 4.4:

Solution 4.5: