WAVES AND SOUND (Chapter # 12) Physics 10th - Question Answers

 Physics 10th - Question Answers

WAVES AND SOUND

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Q.1: Define the following terms:

• Wave
• Wave motion
• Oscillation or vibration
• Oscillatory or vibratory motion
• Periodic motion
• Displacement
• Amplitude
• Time period
• Frequency
• Wavelength

Ans:

1. WAVE:

   • A mechanism in which energy is transferred from one point to another point and the molecules of the system do not change their position is called a wave.

2. WAVE MOTION:

   • A mechanism in which disturbance is transferred from one point to another point and the molecules of the system do not change their position is called wave motion.

3. OSCILLATION OR VIBRATION:

   • A complete round trip about the mean position or central position or equilibrium position is called oscillation or vibration.

4. OSCILLATORY OR VIBRATORY MOTION:

   • When a moving body moves to and fro about the mean position, then it is called oscillatory or vibratory motion.

5. PERIODIC MOTION:

   • A motion which repeats itself in equal intervals of time is called periodic motion.

6. DISPLACEMENT:

   • The minimum distance covered by the moving body from its mean position is called displacement. It is denoted by $X$. Its unit is meter.

7. AMPLITUDE:

• The maximum distance covered by the moving body from its mean position is called amplitude. It is denoted by $X_0$. Its unit is meter.

TIME PERIOD:

• The time taken by the moving body to complete one round trip about the mean position is called time period. It is denoted by $T$. Its unit is second.

FREQUENCY:

• The number of vibrations in one second is called frequency. It is denoted by $f$. Its unit is hertz (Hz).

WAVELENGTH:

• It is the distance between two consecutive crests or troughs. It is denoted by $\lambda$. Its unit is meter.

Q.2: Define frequency? Write down its formula and unit?

Ans: FREQUENCY:

• The number of vibrations per unit time is called frequency.
  • OR
• The number of waves per unit time is called frequency.

It is denoted by $f$.

Formula:

$$f = \frac{1}{T}$$

Or

$$f = \frac{\text{number of waves}}{\text{time}}$$

Or

$$f = \frac{\text{number of vibrations}}{\text{time}}$$

Unit:

• The unit of frequency is cycle/second or hertz (Hz).

Q.3: Define Simple Harmonic Motion? Write down its conditions and give some examples?

Ans: SIMPLE HARMONIC MOTION (S.H.M):

• The type of oscillatory motion in which the acceleration of a body is directly proportional to its displacement and the acceleration is always directed towards the mean position is called Simple Harmonic Motion (S.H.M).

Conditions for S.H.M:

1. There must be elastic restoring force acting on the system.
2. The system must have inertia.
4. The acceleration of the system should be proportional to its displacement from the mean position.
5. The acceleration of the system is always directed towards the mean position.

Examples:

• The motion of a bob of a pendulum.
• The motion of a swing.
• The motion of the string of a sitar.

Q.4: Prove that the motion of a body attached with a spring and placed on a smooth surface is simple harmonic motion?

Ans: Consider a body of mass “$m$” attached to the free end of an elastic spring whose other end is fixed. If the body is displaced from its equilibrium position and then released, it will start vibrating under the action of the restoring force of the spring. This force produces acceleration in the body. Suppose the body is at a distance “$x$” from the equilibrium position at any instant, then the restoring force acting on it to bring it back to its initial position is given by:

$$F = -k x \quad \text{--- (i)}$$

According to Newton’s 2nd law of motion:

$$F = m a \quad \text{--- (ii)}$$

By comparing equation (i) and equation (ii), we have:

$$m a = -k x$$
$$a = -\frac{k}{m} x$$$$a = -(\text{constant}) x$$$$a \propto -x$$

Q.5: Write down the formula for the time period of a simple pendulum and vibrating mass attached to the spring?

Ans: FORMULA FOR SIMPLE PENDULUM:

We can find the time period of the simple pendulum by using the following formula:

$$T = 2\pi \sqrt{\frac{l}{g}}$$

Formula for the time period of vibrating mass attached to the spring: We can find the time period of vibrating mass attached to the spring as:

$$T = 2\pi \sqrt{\frac{m}{k}}$$

Frequency of spring connected with the body executing S.H.M.: We know that:

$$f = \frac{1}{T}$$

But we know that:

$$T = 2\pi \sqrt{\frac{m}{k}}$$

Therefore:

$$f = \frac{1}{2\pi} \sqrt{\frac{k}{m}}$$

Hence:

$$f = \frac{1}{2\pi} \sqrt{\frac{k}{m}}$$

Q.6: What is spring constant? Write its unit?

Ans: SPRING CONSTANT:

• It is the force required to produce an extension of 1 m in a spring.

Unit:

• Its unit is N/m (Newton per meter).

Q.7: Define Simple Pendulum? Prove that the motion of a Simple Pendulum is S.H.M.

Ans: SIMPLE PENDULUM:

• An ideal simple pendulum consists of a point mass suspended by a weightless and inextensible string from a fixed support.

Explanation:

Consider a simple pendulum. In the beginning, the bob of the pendulum is at point “O”; then it is displaced to “A”. The distance between O and A is “x”.
Then work done = $Fd = Fx$.

And it gains maximum potential energy at “A”. When released, it starts periodic motion around its mean position “O”. It means that potential energy is greater at “A” and “B”. Kinetic energy is maximum at “O” due to maximum velocity. Its velocity decreases at the time when it is moving “O” to “A” or “O” to “B” and velocity decreasing means acceleration is negative, and at extreme positions, it is zero. So, we say that the acceleration of the bob is directly proportional to the displacement in the opposite direction.

$$a \propto -x$$

Hence, the motion of a simple pendulum is S.H.M.

Q.8: Define the following terms?

• Natural frequency
• Forced vibration
• Resonance

Ans:

1. NATURAL FREQUENCY:

   • The frequency of a vibrating body by which it vibrates when it is left undisturbed after being set into vibratory motion.

2. FORCED VIBRATION:

   • If a body, vibrating with its natural frequency, is placed in contact with a second body, the latter will also be forced to vibrate at the same frequency as the former. Then this vibration of the second body is called forced vibration.

3. RESONANCE:

   • Whenever the frequency of a vibrating body acting on a system coincides with the natural frequency of the system, then the induced or forced vibration has a very large amplitude. This unique case of forced vibration is called “resonance”.

Q.9: Demonstrate the phenomenon of resonance?

Ans: DEMONSTRATION OF RESONANCE:

• Consider a long string or a metallic wire, stretched tightly between two pegs. Four pendulums A, B, C, and D of different lengths are fastened to the string or wire.
• Another pendulum E of the same length as that of B is also fastened.
• When pendulum E is set swinging, it will be observed that all pendulums start swinging, but pendulum B begins to vibrate with an increasingly larger amplitude.
• As pendulum E is set into vibration, it imparts its motion to the string or metallic wire. This string in turn imparts the same periodic motion to the pendulum tied to it.

Natural frequency of all other pendulums except pendulum B being different, they do not respond to the same extent to the motion imparted from the string.

• Pendulum B responds as its natural frequency agrees with the frequency of the motion of the string, which in turn was supplied by the vibrating pendulum E.
• This phenomenon under which pendulum B begins to vibrate is called resonance.

Q.10: Give any two examples of resonance?

Ans: EXAMPLES:

1. An interesting example of resonance is that of a swing. While enjoying, we apply force by the special movement of our body at a particular position in every vibration. The result is an increase in the amplitude of the swing.
2. While crossing a hanging bridge, soldiers are ordered not to march in step but to break their steps. The reason is that the bridge receives periodic impulses by regular footsteps of a marching column of soldiers. The time period of the periodic impulses happens to be equal to the natural time period of the bridge, and the amplitude will be increased, causing the bridge to collapse.

Q.11: What is wave motion? OR What do you understand by wave motion?

Ans: WAVE MOTION: Wave motion is the form of disturbance that travels through a medium due to periodic motion of particles of the medium about their mean position. A wave can transfer energy from one point to another. All waves have different velocity, wavelength, and frequency.

Q.12: Describe the phenomenon of waves with the help of an experiment?

Ans: FIRST EXPERIMENT:

• If we dip a pencil into a tub of water and take it out, a pronounced circular ripple is set up on the water surface and travels towards the edges of the tub. However, if we dip the pencil and take it out many times, a number of ripples will be formed one after the other. These are shown in the figure.

If you place a small object on the water, it moves up and down when a wave passes across its position; it does not move outward as the waves do. This shows that the disturbance proceeds as water travels outward from the center of disturbance, while the water itself does not move outward. Such up-and-down movements are vibrations of water, which constitute waves and are an example of wave motion.

SECOND EXPERIMENT: Waves can also be produced on very long ropes or strings. If one end of the rope is fixed and the other end is given a sudden up-and-down jerk, a pulse-shaped wave is formed, which travels along the rope.

If, however, jerking of the rope is continued, a number of wave pulses will be produced. This is called a wave train, and it travels along the rope. From the above observation, it may be concluded that a wave is a traveling disturbance.

Q.13: What are the two basic types of waves? Define these types with examples?

OR

Define the following and give examples?

• Transverse wave
• Longitudinal wave

Ans:

1. TRANSVERSE WAVE:

   • The waves in which the particles of the medium execute simple harmonic motion and travel in the direction perpendicular to the propagation of waves are known as transverse waves. They consist of crests and troughs.

   Examples of Transverse Waves:

   • Light waves
   • Microwaves
   • Radio waves

2. LONGITUDINAL WAVES (OR COMPRESSION WAVES):

• The waves in which the particles of the medium execute S.H.M. along the line parallel to the propagation of the waves are known as longitudinal or compression waves.Longitudinal waves consist of compressions and rarefactions. The propagation of longitudinal waves takes place due to the elasticity and inertia of the medium.

Examples of Longitudinal Waves:

• Sound waves
• Seismic waves

Q.14: What are the characteristics of waves?

Ans: CHARACTERISTICS:

1. A vibrating body and a material medium are necessary for the production of waves.
2. When waves pass through a medium, the particles of the medium start vibrating at their own positions.
3. The velocity of a wave is equal to the product of wavelength and frequency, i.e., $v = f\lambda$.
4. The velocity of a wave is entirely different from the velocity of the vibrating particles of the medium.

Q.15: Find a relation between velocity, frequency, and wavelength?

Ans: We know that a particle of a medium completes one vibration as one wave passes through it. This means that a wave travels a distance equal to one wavelength during the time period $T$. If the wavelength is $\lambda$, then the velocity of the wave is given as:

$$\text{Velocity} = \frac{\text{Distance}}{\text{Time}}$$ $$v = \frac{\lambda}{T}$$

Or

$$v = \frac{1}{T} \times \lambda$$

But we know that:

$$f = \frac{1}{T}$$

Thus:

$$v = f\lambda$$

Q.16: Define reflection of waves? What do you understand by interference of waves? What do you understand by constructive and destructive interference?

Ans: REFLECTION OF WAVES:

• Bouncing back of the waves into the same medium after striking a barrier or obstacle is called reflection of waves.
• The waves which hit the barrier are called incident waves.
• The waves which originate from the barrier are called reflected waves.

INTERFERENCE OF WAVES:

• Interaction of two waves passing through the same region of space at the same time is called interference of waves.

TYPES OF INTERFERENCE: There are two types of interference of waves:

1. Constructive interference
2. Destructive interference

Constructive Interference:

• If the interference of two waves results in a wave of greater amplitude, the interference is called constructive interference.

Destructive Interference:

• If the interference of two waves results in a wave of zero amplitude, the interference is called destructive interference.

Q.17: Define the following?

• Travelling or Progressive wave
• Stationary or Standing wave

Ans:

1. TRAVELLING OR PROGRESSIVE WAVE:

   • Waves or disturbances which move from one place to another along the medium are called travelling or progressive waves. They have certain velocity, and they can transfer energy, e.g., sound waves, light waves, electromagnetic waves.

2. STATIONARY OR STANDING WAVE:

   • When two waves of equal amplitude, frequency, and wavelength travelling through the same medium in opposite directions meet one another, the result is a wave which does not travel in either direction. Such waves are called stationary waves. They cannot transfer energy from one point to another point.
   • They have nodes and antinodes, e.g., wave in a string tied with two ends, wave in a string of sitar and violin.

Q.18: Define sound. How is sound produced?

Ans: SOUND:

• Sound is a form of energy. Sound is the sensation provided by the ear.

How Sound Is Produced:

• Sound is produced by a vibrating body.
• The sound is produced when a body vibrates at least twenty times in a second in order to produce audible sound.

Q.19: What are the factors on which propagation of sound depends?

Ans: FACTORS: There are three factors on which the propagation of sound depends:

1. A vibrating body with a proper frequency
2. A material medium
3. A listener

Q.20: Show by experiment that a medium is necessary for sound waves?

Ans: MEDIUM IS NECESSARY FOR PROPAGATION OF SOUND:

• Suspend an electric bell in a jar by its wires through a cork fixed in its mouth. Switch on the bell. We will hear the sound of the bell. Now start removing air from the jar with the help of an exhaust pump.
• The loudness of the sound of the bell will start decreasing ultimately, although the hammer is still seen striking the bell. This experiment shows that air is necessary for the propagation of sound; in fact, a material medium such as air, water, metal, etc., is required.

Q.21: Define the following terms?

• Audible sound
• Infrasonic sound
• Ultrasonic sound

Ans:

1. AUDIBLE SOUND:

   • Sound waves which have frequency between 20 Hz to 20,000 Hz are called audible sound because they can be heard by the human ear.

2. INFRASONIC SOUND:

   • The frequency of sound waves less than 20 Hz is known as infrasonic sound.

3. ULTRASONIC SOUND:

   • The frequency of sound waves greater than 20,000 Hz is called ultrasonic sound.

Q.22: How can the velocity of sound be measured? Describe in detail.

Ans: VELOCITY OF SOUND:

• The distance traveled by the sound waves in unit time is known as the velocity of sound.

Experiment:

For this experiment, select two stations at a distance of 8 to 10 km such that there is no obstacle between them which can hinder the view. Fire a gun at station A and ask your friend at station B to start a stopwatch on seeing the flash. The stopwatch should be stopped on hearing the sound of the gun.

In this way, the time taken by the sound to travel from station A to station B is measured. Now fire a gun at station B and repeat the above process so that if there is any possible error in measuring the velocity of sound due to the direction of wind, then it can be removed.

Calculate the mean time. The distance $S$ between the two stations is already known. Put the value in the formula and calculate the exact value of velocity of sound.

$$t_{av} = \frac{t_1 + t_2}{2}, \quad v = \frac{S}{t_{av}}$$

Q.23: Define musical sound and noise?

Ans:

1. MUSICAL SOUND:

   • A sound which produces a pleasant sensation upon the ear is called a "musical sound." It has a definite frequency caused by regular and periodic vibration.

2. NOISE:

   • A sound which produces an unpleasant and jarring sensation upon the ear is called "noise." It has no definite frequency. It is a discontinuous sound produced by a sort of confused, sharp, and irregular vibration in a medium.

Q.24: What are the characteristics of a musical sound?

Ans: CHARACTERISTICS OF A MUSICAL SOUND: There are three characteristics of musical sound which are as follows:

1. Loudness:
• It is the property of all sounds. It depends upon the intensity of the sound waves. Intensity of sound waves is defined as the energy carried by the sound waves through a unit area placed perpendicular to the direction of propagation of waves per second. Loudness enables us to distinguish between a faint and a loud sound.

2. Pitch:

• The characteristic of sound by which we can distinguish between flat and shrill sound is called pitch of the sound. It depends upon the frequency of the vibrating body. The greater the frequency, the higher will be the pitch of the sound.

3. Quality or Timbre:

• The characteristic of sound by which we can distinguish between two sounds of the same pitch and loudness is called quality or timbre of the sound.

Q.25: Write down the factors on which the loudness of sound depends?

Ans: FACTORS ON WHICH THE LOUDNESS OF SOUND DEPENDS: Loudness of the sound depends on the following factors:

1. Amplitude of Motion of Vibrating Object:

   • The loudness of sound is directly proportional to the square of the amplitude of the sound-producing waves.

2. Distance of a Source:

   • The loudness of sound is inversely proportional to the square of its distance from the source. So, the smaller the distance, the louder the sound.

3. Area of Vibrating Body:

   • Loudness of sound is directly proportional to the area of the vibrating body.

4. Direction of Wind:

   • If the sound waves travel in the direction of the wind, a loud sound is heard. But if the sound is traveling against the wind, a faint sound will be heard.

5. Density of Medium:

   • Loudness of sound also depends upon the density of the medium through which the sound is traveling. Thus, the larger the density, the louder is the sound. If the density is less, a faint sound is heard.

Q.26: What is echo? What is the minimum distance of the sounding body from the obstacle to hear an echo?

Ans: ECHO:

• The sound heard after reflection from an obstacle is known as echo.

The Minimum Distance of the Sounding Body from the Obstacle to Hear Echo:

• If the distance between the source and obstacle is $d$, then the total distance traveled by the sound after reflection from the obstacle is $2d$. Let the time taken to hear the echo be $t$ seconds.

$$\text{Distance} = V \times t$$

$$2d=V\times t$$

where $v$ is the velocity of sound. This distance must be covered in 0.1 seconds or more so that the echo is heard.

Hence:

$$2d = v \times \frac{1}{10}$$

The velocity of sound in air at $15^{\circ} \text{C}$ is about $340 \, \text{m/s}$, then the above equation will become:

$$2d = 340 \times \frac{1}{10}$$ $$2d = 34$$ $$d = 17 \, \text{meters}$$

It means the minimum distance of the sounding body from the obstacle to hear an echo should be about $17$ meters.

Q.27: Define reflection of a sound wave with the help of an experiment? Give some examples?

Ans: REFLECTION OF SOUND:

• Coming back of sound waves into the same medium after striking an obstacle is called reflection of sound.

Experiment:

• Take a long PVC pipe and cut it into two equal parts. Hold the two parts against a smooth surface. Place a watch at the open end of one tube and ask a student to place his ear against the open end of the second tube. Tell the students to slightly move the tube sideways until clear ticking of the watch is heard. Place a big cardboard sheet between the two tubes so that the sound does not reach the ear through any other path. Measure the angles that the two tubes make with the normal at the point of incidence as shown in the figure. Repeat the experiment by changing the angle of incidence.

Examples:

• The whispering gallery in the Shah Jehan Mosque, Thatta.
• The whispering tube.
• Stethoscope.

Q.28: What do you understand by the reflection of a sound wave?

Ans: REFLECTION OF SOUND WAVE:

• If a sound wave collides with a smooth surface, it reflects. The sound wave falling on the surface is called the incidence wave, which makes an angle with the normal. This angle is called the angle of incidence.
• The sound wave reflecting back from the surface is called the reflected wave. It makes an angle with the normal called the angle of reflection. The angle of incidence and the angle of reflection will be the same.

Q.29: Define interference of sound? Explain its two types?

Ans: INTERFERENCE OF SOUND:

• The phenomenon in which two sound waves of the same frequency and amplitude passing through the same region of space at the same time interfere with each other is called interference of sound.

Types of Interference:

1. Constructive Interference:

   • When compression of one sound wave falls on the compression of the second wave or rarefactions of the two waves coincide, the louder sound is heard, and the interference is called constructive interference.

2. Destructive Interference:

   • If the compression of one wave falls on the rarefaction of the second wave, we hear no sound or a very faint sound. This type of interference is called destructive interference.

Q.30: Demonstrate interference of sound by an experiment?

Ans: DEMONSTRATION OF INTERFERENCE OF SOUND:

• Two loudspeakers $X$ and $Y$ are connected to the same signal generator (tuner or amplifier).
• This signal generator is set at a frequency of about 3000 Hz.
• The distance between the loudspeakers is about 0.5 m.
• When the generator is switched on, the two loudspeakers produce sound.
• If we stand about 2 m away from the speakers and after blocking one ear, move our head sideways through at least 0.2 m, we will hear variation in loudness.
• A microphone may be used to detect the change in the intensity (loudness) of the sound by moving it along the line $AB$, which is about 2 m away from the loudspeakers. The microphone detects the rise and fall of the loudness of the sound produced by the loudspeakers. This shows interference of sound waves does take place.

Q.31: What do you understand by resonance of sound?

Ans: RESONANCE OF SOUND:

• When the natural frequency of an air column matches with the frequency of incident sound, as a result of which a loud sound is heard, this phenomenon is called resonance of sound.

Q.32: Demonstrate the resonance of sound by an experiment?

Ans: DEMONSTRATION:

• A simple apparatus for demonstrating the resonance of sound is shown in the figure.
• A long vertical tube is partially dipped in water contained in a beaker.
• A vibrating tuning fork is held near the upper end of the tube.
• The length of the air column is adjusted vertically by moving the tube out of the water.
• The sound waves generated by the tuning fork are reinforced when the length of the air column corresponds to one of the resonant frequencies of the tube.
• The arrangement can be used to determine the velocity of sound in air.
• Whenever a sound wave comes across a barrier, it is reflected back in the same medium.
• In this process, the reflected waves interact with the incident waves and produce stationary waves.
• When this happens, we get a louder sound.
• The loud sound indicates that the reflected waves are in resonance with the incident waves produced by the tuning fork.
• The speed of sound can be calculated by $v = f\lambda$, where the wavelength ($\lambda$) is four times the distance at which the maximum loudness is obtained.

Q.33: What do you understand by beats?

Ans: BEATS:

• The periodic variation in intensity of sound at a given point due to the superimposition of two waves having slightly different frequencies is called beats. Beats are produced because of the interference of sound waves of slightly different frequencies.
• The number of beats one hears per second is called beat frequency. It is equal to the difference in frequency between the two sounds. The maximum beat frequency that a human ear can detect is 7 beats per second.

Q.34: Define ultrasonic waves? Write down its characteristics and applications?

Ans: ULTRASONIC WAVES:

• Ultrasonic waves are longitudinal waves with frequencies above the audible range. They can be produced by setting a quartz crystal to oscillate electrically. Ultrasonic waves of frequencies of the order of 10 Hz or more can be produced with such a device.

Characteristics:

1. Their wavelength is much shorter than normal sound waves.
2. They can penetrate deeper into the sea.

Application:

• To examine the soft fleshy parts of the body.
• To obtain cross-sectional pictures of patients.
• To make ultrasound guidance devices for the blind.
• They can be used in echo-depth sounding devices to determine the depth of the sea floor.
• To detect cracks in metal structures.
• To kill bacteria and microorganisms in liquids.
• To clean places that cannot be cleaned in a normal way.
• To clean delicate instruments and materials such as jewelry.

Q.35: Why is the explosive sound produced in the sun not heard on the earth?

Ans:

• As we know, a vast vacuum is present (i.e., there is no medium between the sun and earth). Sound waves cannot travel without a medium. So, the explosive sound produced in the sun is not heard on the earth.

Q.36: Why is the flash of lightning seen earlier than the sound of thunder?

Ans:

• As we know that the speed of light is greater than the speed of sound, so the light reaches us first due to greater velocity than sound.

Q.37: Write down the difference between musical sound and noise?

Musical Sound	Noise
It produces a pleasant effect.	It produces an unpleasant effect.
It has some regularity.	It is an abrupt sound.
It depends upon pitch quality and loudness of sound.	It depends upon intensity of sound.

Q.38: Write down the difference between transverse waves and longitudinal waves?

Transverse Waves	Longitudinal Waves
They may be matter or electromagnetic in nature.	They are only mechanical waves.
In these waves, the particles of the medium execute S.H.M perpendicular to the direction of waves.	In these waves, the particles of the medium execute S.H.M along the direction of the waves.
The position above the mean line is called crest.	The region where the crowding of the particles of the medium is greater is called compression.
The distance between the two consecutive troughs is called wavelength.	The distance between the two consecutive compressions is called wavelength.