CBSE Class 9 Physics Waves and Sound Worksheets Set 04

Short and Long Answer Questions on Waves and Sound

Question 1. Can you hear your own words if you are standing in a perfect vacuum? Can you hear your friend in the same conditions?
Answer:
No, in neither case.
Sound is a mechanical wave and needs a material medium (solid, liquid or gas) to travel. In a perfect vacuum there is no medium, so sound can neither reach your own ears nor your friend's.
In simple words: No medium, no sound.

Teacher's Note:
1. Radio waves, being electromagnetic, could still be used to communicate.

Question 2. A vertical rod is hit at one end. What kind of wave propagates in the rod if (a) the hit is made vertically, (b) the hit is made horizontally?
Answer:
(a) The hit is along the length of the rod, so the particles vibrate along the direction of travel: a longitudinal wave.
(b) The hit is perpendicular to the length of the rod, so the particles vibrate across the direction of travel: a transverse wave.
In simple words: Along the rod gives longitudinal; across the rod gives transverse.

Teacher's Note:
1. Solids can carry both kinds of wave.

Question 3. Two loudspeakers are arranged facing each other at some distance. Will a person standing behind one of the loudspeakers clearly hear the sound of the other loudspeaker, or will the clarity be seriously damaged because of the 'collision' of the two sounds in between?
Answer:
The person hears the other loudspeaker clearly.
Waves pass through each other and continue unchanged after crossing (superposition). The two sounds only combine at the places where they overlap and do not damage each other.
In simple words: Waves do not collide; they pass through each other.

Teacher's Note:
1. Overlapping waves add up and separate again.

Question 4. Explain why: (a) The velocity of sound is generally greater in solids than in gases. (b) The velocity of sound in oxygen is less than in hydrogen.
Answer:
(a) v = √(E/ρ). Solids have a very large elasticity E compared with gases, which outweighs their higher density, so sound travels faster in solids.
(b) At the same temperature the speed of sound in a gas is inversely proportional to the square root of its molar mass. Oxygen molecules are 16 times heavier than hydrogen molecules, so the speed is lower in oxygen (about 4 times lower).
In simple words: Elasticity and density decide the speed.

Teacher's Note:
1. v is larger for stiffer and lighter media.

Question 5. The thunder of lightning is heard some moments after the flash is seen. Explain.
Answer:
Light travels at about 3 × 10⁸ m/s, which is nearly instantaneous over the distance to the cloud.
Sound travels at only about 340 m/s in air, so it takes a noticeable time to reach us. Hence the flash is seen first and the thunder is heard later.
In simple words: Light is much faster than sound.

Teacher's Note:
1. The time gap can be used to estimate how far the lightning is.

Question 6. How can people communicate with each other on the moon's surface? Give reason.
Answer:
They cannot talk directly, because the moon has no atmosphere and sound needs a medium to travel.
They communicate with radio (electromagnetic) waves using wireless sets, because electromagnetic waves travel through vacuum.
In simple words: Use radio waves instead of sound.

Teacher's Note:
1. Astronauts use radio sets in their suits.

Question 7. Will the time period of a pendulum vary from the pole to the equator of the earth? Give reason.
Answer:
Yes. T = 2π√(L/g).
The value of g is greater at the poles than at the equator, so the time period is slightly shorter at the poles and longer at the equator.
In simple words: T depends on g, and g changes with latitude.

Teacher's Note:
1. Earth is flattened at the poles, so g is larger there.

Question 8. Can you make a pendulum oscillate at the centre of the earth? Give reason.
Answer:
No. At the centre of the earth g = 0, so there is no restoring force on the bob and no oscillation takes place (T = 2π√(L/g) becomes infinite).
In simple words: With no weight there is nothing to bring the bob back.

Teacher's Note:
1. The restoring force of a pendulum comes from gravity.

Question 9. No sound energy from the sun reaches the earth. Can you conclude that the sun does not produce any sound energy?
Answer:
No. The sun may produce sound, but sound needs a material medium and there is a vacuum in space between the sun and the earth, so the sound cannot reach us.
In simple words: Sound cannot cross empty space.

Teacher's Note:
1. Light and heat reach us because they are electromagnetic waves.

Question 10. Write two differences between a sound wave and an electromagnetic wave.
Answer:
1. Sound is a mechanical wave and needs a medium; an electromagnetic wave needs no medium and travels in vacuum.
2. Sound in air is longitudinal, whereas electromagnetic waves are transverse. Also, sound travels at about 340 m/s in air but electromagnetic waves travel at 3 × 10⁸ m/s in vacuum.
In simple words: Medium, type of wave and speed are the differences.

Teacher's Note:
1. Remember: sound cannot travel in vacuum.

Question 11. When vapour increases in the atmosphere, what happens to the speed of sound? Give your answer with reason.
Answer:
The speed of sound increases.
Water vapour is lighter than dry air, so humid air has a smaller density, and the speed of sound is inversely related to the square root of the density.
In simple words: Moist air is less dense, so sound travels faster.

Teacher's Note:
1. Sound travels faster in humid air than in dry air at the same temperature.

Question 12. What is called reverberation? How is it reduced in a small room?
Answer:
Reverberation is the persistence of sound in a room because of repeated reflections from the walls, floor and ceiling.
It is reduced by covering the walls and ceiling with sound-absorbing materials such as curtains, carpets, fibreboard and cushions.
In simple words: Repeated reflections prolong sound; soft materials absorb it.

Teacher's Note:
1. In big halls the roof is covered with sound absorbers.

Question 13. In a class room why don't we receive any echo?
Answer:
An echo is heard only if the reflected sound reaches the ear at least 0.1 s after the original, which needs a reflecting surface at least about 17 m away.
The walls of a classroom are much closer, so the reflected sound reaches the ear almost at once and merges with the original sound.
In simple words: The walls are too near for the echo to be separate.

Teacher's Note:
1. The minimum distance for an echo is about 17 m.

Question 14. What causes the change in velocity when a sound wave changes medium while propagating?
Answer:
The speed of sound depends on the elasticity and the density of the medium. When the wave enters a different medium these two properties change, so the speed changes.
In simple words: Different media have different elasticity and density.

Teacher's Note:
1. The frequency stays the same; the wavelength changes.

Question 15. What are longitudinal waves? Give examples. How do such waves propagate?
Answer:
In a longitudinal wave the particles of the medium vibrate back and forth along the direction in which the wave travels.
Examples: sound in air, waves in a stretched spring when pushed and pulled along its length.
Propagation: a vibrating source pushes the layer next to it, making a compression (high pressure). The compressed layer expands and compresses the next layer. Then the source moves back, leaving a rarefaction. The series of compressions and rarefactions travels forward while each particle only moves about its position.
In simple words: Compressions and rarefactions travel along the direction of vibration.

Teacher's Note:
1. Draw the compressions (C) and rarefactions (R) in your answer.

Question 16. Two children are at the opposite ends of an iron pipe. One strikes the end of the pipe with a stone. Find the ratio of the time taken by sound waves in air and in the iron pipe to reach the other child. The velocities of sound in air and iron are 332 m/s and 5,130 m/s respectively.
Answer:
For the same distance d, t = d/v.
t_air / t_iron = v_iron / v_air = 5130/332 ≈ 15.45.
The ratio is about 15.5 : 1.
In simple words: Time is inversely proportional to speed for the same distance.

Teacher's Note:
1. Sound reaches through the iron pipe first.

Question 17. A second's pendulum (on earth) is taken to the moon. What will be its time period there? (g on the moon is one-sixth of that on earth.)
Answer:
A second's pendulum has T = 2 s on earth.
T ∝ 1/√g, so T_moon = 2 × √6 = 4.9 s.
The time period on the moon is about 4.9 s.
In simple words: Smaller g means a longer period, √6 times as long.

Teacher's Note:
1. T = 2π√(L/g).

Question 18. A man standing on a boat observes that every 5 seconds one crest strikes his boat. Two such crests are 1 m apart. In how much time will 100 such crests strike his boat? Find the velocity of the wave.
Answer:
Time period T = 5 s and wavelength λ = 1 m.
Velocity v = λ/T = 1/5 = 0.2 m/s.
Time for 100 crests = 100 × 5 = 500 s.
In simple words: One crest every 5 s, so 100 crests take 500 s.

Teacher's Note:
1. v = λ/T = fλ.

Question 19. Describe an experiment to demonstrate that a wave does not carry any matter but carries energy.
Answer:
Drop a stone into a pool of still water and place a small cork on the surface.
Ripples spread outward in circles, but the cork only moves up and down at its place and does not travel with the ripples.
This shows that the disturbance (energy) moves forward while the water (matter) stays in place; the moving ripples can also do work, for example by pushing a floating leaf up and down.
In simple words: The cork only bobs; the wave moves on.

Teacher's Note:
1. Waves transfer energy, not matter.

Question 20. In a certain planet the gravitational acceleration is 3 times more than on the earth. Find the ratio of the time periods in the two planets.
Answer:
T = 2π√(L/g), so T ∝ 1/√g.
T_earth / T_planet = √(g_planet / g_earth) = √3.
T_earth : T_planet = √3 : 1 (about 1.73 : 1).
In simple words: A bigger g shortens the period.

Teacher's Note:
1. Take the square root of the ratio of g.

Question 21. A man standing between two hills makes a sound loudly and receives two echoes after 1 s and 1.2 s. Find the distance between the two hills. (Take the speed of sound as 330 m/s.)
Answer:
Distance of the first hill = 330 × 1 / 2 = 165 m.
Distance of the second hill = 330 × 1.2 / 2 = 198 m.
Distance between the hills = 165 + 198 = 363 m.
In simple words: Each echo covers twice the distance to its hill.

Teacher's Note:
1. Echo time is the time to go and return.

Question 22. A boy standing at a certain distance from a hill shouts loudly and starts running towards the hill at a speed of 5 m/s. He receives the echo after 5 seconds. Find the distance of the boy at the initial position from the hill. (Take the speed of sound as 330 m/s.)
Answer:
Let the initial distance be d. In 5 s the boy runs 5 × 5 = 25 m, so the distance he still has to the hill is d − 25.
The sound travels to the hill and back to the boy: d + (d − 25) = 330 × 5 = 1650.
2d = 1675, so d = 837.5 m.
In simple words: The sound covers d to the hill and d − 25 back, totalling 1650 m.

Teacher's Note:
1. Account for the distance the boy moves.