## Kerala Plus One Physics Chapter Wise Questions and Answers Chapter 15 Waves

**Very Short Answer Type Questions (Score 1)**

Question 1.

When a wave advances through a medium it carries

a. The particles of the medium

b. energy

c. sound

Answer:

b. energy

Question 2.

The sound wave propagation in air is ……

Answer:

a.longitudinal

Question 3.

“When a stone is dropped into the surface of the still water, the waves produced are longitudinal”. State whether this statement s true or false?

Answer:

False. The waves produced are transverse.

Question 4.

When a tuning fork produces sound waves in air, which one of the following properties of sound is same in the material of tuning fork as well as air?

a. wavelength

b. frequency

c. velocity

Answer:

b. frequency

Question 5.

When the source of sound approaches the listener the apparent frequency of sound will be ……… (higher / smaller)

Answer:

a. higher

Question 6.

“The velocity of sound in vacuum is zero”. State whether this statement is true or false?

Answer:

True

Question 7.

When a tuning fork of frequency 256 Hz is excited together with another fork, three beats are heard per second. Then the frequency of other fork is 256 Hz

a. 253 Hz

b. 259 Hz

c. 253 or 259 Hz

Answer:

c. 253 Hz or 259 Hz (V_{beat} = V_{1} – V_{2})

Question 8.

Ultrasonic signal sent from a SONAR returns to it after reflection from a rock after a lapse of 1 second. If the velocity of ultrasound in water is 1600 m/s, the depth of the rock in water is …….

a. 300 m

b. 400 m

c. 800 m

Answer:

c. 800 m (2 × depth = speed)

Question 9.

If L is the length of a closed pipe containing gas which vibrates at the fundamental mode, then the wavelength of the wave is

a. 2L

b. L/2

c. 4L

d. 3L

Answer:

c. 4L (fundamental mode L = λ/4) .

Question 10.

Two sitar strings A and B are slightly out of tune and produce beats of frequency 6 Hz. The tension in the string A is slightly reduced and the beat frequency is found to be 3 Hz. If the original frequency of A is 324 Hz, what is the frequency of B ?

Answer:

318 Hz

**Short Answer Type 9 Questions (Score 2)**

Question 1.

a. A stationary sound wave has frequency 165 Hz. Speed of sound in air is 330 m/s. Then distance between two consecutive nodes is

i. 2m

ii. 1m

iii. 0.5

b. λ_{1} =100 m, λ_{2} = 90 cm and velocity = 396 m/s. When λ_{1} and λ_{2} superimpose, the number of beats are

i. 4

ii. 40

iii. 44

iv. 400

Answer:

a. ii. 1m (distance between consecutive

nodes = λ/2 and λ = 330/165)

b. i. 44 (f_{1} = 396/1 = 396 Hz,

f_{2} = 396/0.9 = 440, no. of beats =f_{2}-f_{1} )

Question 2.

Table below gives the speed of sound in some media at ordinary temperatures. Medium

Speed

Medium | Speed |

X | 343 |

Y | 1482 |

Z | 3560 |

b.

a. Identify X, Y and Z from the following list.

Air, copper, water, vulcanized rubber

b.If ‘B’ is the bulk modulus and ‘p’ is the density of a medium, give the equation of speed of longitudinal wave in a medium.

Answer:

a.

Medium | Motion |

X | Air |

Y | Water |

Z | Copper |

Question 3.

a. Draw an expected graph between the pressure and the speed of sound waves passing through gas.

b. Why sound waves cannot travel in vacuum while light waves can?

Answer:

a.

b. Sound waves require medium for their propagation

Question 4.

a. Two sound waves in air have wave length 1 : 3. Find their frequency ratio.

b. Which has greater velocity-mechanical waves or electro megnetic waves

.

Answer:

b. Electromagnetic waves

**Short Answer Type Questions (Score 3)**

Question 1.

Consider a string stretched between two fixed supports. A stationary wave can be produced on the string by suitably exciting with a tuning fork.

a. What is a stationary wave?

b. Explain conditions for formation of a stationary wave.

c. Draw a figure showing the second harmonic of vibration of stretched string fixed at both ends.

Answer:

a. The stationary waves are not progressive. The condensations (crests) or rare fractions (troughs) do not travel forward or backward.

b. The interference of two identical waves moving in the opposite direction produce stationary waves.

Question 2.

In a resonance column apparatus, the air contained in the tube is set into vibrations by holding an excited tuning fork at its mouth.

a. Identify resonance column apparatus as an open pipe or closed pipe.

b. Write the expression for frequency of nth mode of vibration of resonance column apparatus.

c. The length of air colum in a resonance column apparatus is 20 cm. Which harmonic mode of the pipe is resonantly excited by a 430 Hz tuning fork (speed of sound in air is 340 m/s)?

Answer:

a. Closed pipe.

Frequency of tuning fork is close to fundamental frequency. So first harmonic will be excited.

Question 3.

A wave function can be represented by

letters have usual meaning

a. What type of the wave is this if the wave advances the medium?

b. What are its basic properties?

Answer:

a. Progressive wave

b.

- The wave advances through the medium
- Each particle in the medium executes same type of vibrations.
- No particle remains permanently at rest
- The particles attain the same maximum velocity when they pass through their mean position.
- Energy transfer takes place during propagation and there is no transfer of matter.

**Long Answer Type Questions (Score 4)**

Question 1.

The following are examples of waves in musical instruments.

i. Vibration of air column in an organ pipe.

ii. Vibration of the string of a violin

iii. Vibration of the membrane of a table.

iv. Vibration of air column in resonance column

a. Classify the following into transverse waves and longitudinal waves.

b. Transverse waves are not produced in liquids and gas. Why?

Answer:

Transverse waves | Longitudinal waves |

Vibration of string of a violin | Vibration of air column in an organ pipe |

Vibration of membrane of tabla | Vibration of air comumn in resonance column |

b. Transverse waves are in the form of crests and troughs involving change in shape. Liquids and gases do not possess the shape.

Question 2.

“When a train travelling at a high speed with its whitstle blowing, approaches to you on the platform, the pitch of the sound whitstle appears to rise”.

a. Which scientific phenomenon is behind it?

b. State that phenomenon.

c. A person is standing on a railway platform. An engine while approaching the platform blows a whitstle of pitch 660Hz. The speed of engine is 72 km /n. Calculate the apparent pitch of the whistle as heard by the person. Velocity of sound = 340 ms^{-1}.

Answer:

a. Doppler effect

b. The phenomenon of apparent in frequency of sound heard by a listener when there is relative motion between the source, observer and the medium in between them is called Doppler effect.

Question 3.

A string of length L is stretched between two rigid supports. It is plucked at the centre.

a. What type of wave is formed in it?

b. Obtain expression for the frequency of higher harmonies of vibrations of a stretched string.

Answer:

a. Standing waves

Question 4.

State the laws of vibrating string

Answer:

The fundamental frequency (υ) of vibrating of a stretched string is I. inversely proportional to the length (l) of the vibrating segment when tension (T) and mass per unit length (m) remain constant

i.e., when T and m are constant

i.e.,υ × l = a constant, This is called law of length

ii. directly proportional to the square root of the tension (T) when length l and mass per unit length (m) remain constant.

i.e., when I and m are constants.

i.e., a constant, This is called law of tension.

iii. inversely proportional to the square root of the linear density m when length (1) and tension (T) are constant.

i.e., when T and l are constants

i.e., a constant, This is called law of mass.

**Long Answer Type Questions (Score 5)**

Question 1.

A train is approaching the station blowing its siren. A man standing on the platform observes a change in frequency of the sound produced.

a. What is the phenomenon called?

b. Write down an expression for the apparent frequency heard by a stationary observer when the source is approaching him with a velocity u.

c. If the speed of the approaching train is 90 km/h and pitch of the siren heard by the observer is 480 Hz, calculate the true pitch of the siren (velocity of sound = 350 m/s ).

Answer:

a. Doppler effect,

Question 2.

While discussing the propagation of sound through atmospheric air, one argued that the velocity of sound is 280 ms and said that he calculated it using Newton’s formula. But another learner argued that velocity of sound is 330 ms. He justified his argument by saying that he has appled Laplace corrected formula.

a. Write the formula used by the second learner

b. Using the above relation, show that velocity depends on temperature and humidity while is independent of pressure.

c. “Sound can be heard over longer distance on rainy days”. Justify.

Answer:

Since is a constant, ‘V is unaffected by the pressure change.

c. On rainy day density of air is less than dry air and hence speed increases.

**Ncert Questions &Answer**

Question 1.

One end of a U-tube containing mercury is connected to a suction pump and the other end to atmosphere. A small pres. sure difference is maintained between the two columns. Show that, when the Suction pump is removed, the column of mercury in the U-tube executes simple harmonic motion.

Answer:

When both the ends are open to the atmosphere, and the difference in levels of the liquid in the two arms is h, the net force on the liquid column is Ahρg where A is the area of cross-section of the tube and ρ is the density of the liquid. Since restoring force is proportional to h, motion is simple harmonic.

Question 2.

A string of mass 2.50 kg is under tension of 200 N. The length of the stretched string is 20.0 m. The string is struck perpendicular to its length at one end.

a. What type of wave is produced? (transveres wave, longitudinal wave)

b. Differentiate between a transverse and longitudinal wave,

c. Find the speed of the wave produced.

d. How long does the disturbance take to reach the other end?

Answer:

a. Transverse wave.

b. Transverse wave: Constituents of the medium vibrate perpendicular to the direction of wave propagation.

Longitudinal wave: Constituents oscillate along the direction of wave propagation.

Question 3.

A stone dropped from the top of a tower of height 300 m high splashes into the water of a pond near the base of the tower. When is the splash heard at the top given that the speed of sound in air is 340 m/s? (g = 9.8 m/s^{2})

Answer:

Question 4.

Use the formula v = to explain why the speed of sound in air

a. is independent of pressure

b. increases with temperature

c. Increases with humidity

Answer:

Assume ideal gas law: where p is

the density, M is the molecular mass, and T is the temperature of the gas.

This gives , This shows that v is:

a. independent of pressure.

b. increases as

Speed of sound increases with humidity.

Question 5.

A bat emits ultrasonic sound of frequency 1000 kHz in air. The sound meets a water surface. Find

i. wavelength of the reflected sound.

ii. wavelength of the transmitted sound speed of sound in air = 340 m/s,

water = 1486 m/s.

Answer:

Question 6.

The transverse displacement of a string clamped its both ends is given by

y (x, t) = 0.06 sin (2π/3x) cos (120πt) where x and y are in meters and t in seconds. The length of the string is 1.5 m and its mass is 3.0 ×10^{2} kg.

a. Does the function represent a travelling wave or a stationary wave?

b. Interpret the wave as a superposition of two waves travelling in opposite direction. What is the

i. wave length

ii. frequency

iii. speed of each wave

c. Determine the tension in the string.

Answer:

a Stationary wave y (x,t)=2a sin(kx) cos (ωt)

b. Comparing with general equation of a stationary wave,

Question 7.

Given below are some functions of x and t to represent the displacement (transverse or longitudinal) of an elastic wave. State which of these represent:

i. a travelling wave

ii. a stationary wave

iii. none at all

a. y = 2 cos (3x) sin (10t)

b.

c. y = 3 sin (5x – 0.5t) + 4 cos (5x – 0.5t)

d. y = cos x sint + cos2x sin 2

Answer:

a. Stationary wave.

b. Unacceptable function for any wave.

c. Travelling harmonic wave.

d. Superposition of two stationary waves.

Question 8.

A hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in the tissue in which the speed of sound is 1.7 km/s? The operating frequency of the scanner is 4.2 MHz.

Answer:

## Leave a Reply