Frank Brothers Solutions for ICSE Class 9 Physics Chapter 7.1 Sound Production And Propagation Of Sound

Page No: 278

Question 1. Is sound a form of energy?
Answer: Yes, Sound is form of energy.
In simple words: Sound is energy because it can make things move, like vibrating your eardrum so you can hear.

📝 Teacher's Note: Remind students that energy is the "ability to do work." Since sound can move particles in a medium, it is a form of energy.

🎯 Exam Tip: Sound is specifically a form of mechanical energy. Mentioning this can help you score higher.

Question 2. How is sound produced?
Answer: Sound is produced by the vibrating objects.
In simple words: Whenever an object shakes back and forth very fast, it creates a sound.

📝 Teacher's Note: Ask students to touch their throat while speaking; they will feel the vibrations of their vocal cords.

🎯 Exam Tip: "Vibration" is the most important keyword for any question about how sound starts.

Question 3. Can sound travel through a vacuum?
Answer: No, sound cannot travel through vacuum.
In simple words: Sound needs "stuff" like air or water to travel. In empty space, there is nothing to carry the sound waves.

📝 Teacher's Note: Explain the Bell Jar experiment where an electric bell becomes silent once the air is pumped out of the jar.

🎯 Exam Tip: Always state that sound requires a "material medium" to travel.

Question 4. Can sound travel through solids and liquids?
Answer: Yes, sound can propagate through solids and liquids.
In simple words: You can hear sounds underwater or through a wooden door because sound travels through those materials.

📝 Teacher's Note: Use the example of a "string telephone" to show how sound travels through a solid string.

🎯 Exam Tip: Sound actually travels faster in solids and liquids than it does in air.

Question 5. Can sound be produced in a vacuum?
Answer: No, sound cannot be produced in vacuum. It needs a material medium.
In simple words: Even if you hit a drum in space, you wouldn't hear anything because there's no air to carry the vibration to your ears.

📝 Teacher's Note: This is a common trick question. While the object can technically vibrate, "sound" (as a wave) cannot exist without a medium.

🎯 Exam Tip: Focus on the "requirement of a medium" for the existence of sound waves.

Question 6. On what factors does the speed of sound depend?
Answer: Speed of sound depends on the density, pressure, temperature, humidity and Wind.
In simple words: How fast sound moves depends on the type of air, how hot it is, how wet it is, and which way the wind blows.

📝 Teacher's Note: Use the analogy of a car: it drives differently on a hot day, a rainy day, or against a strong headwind.

🎯 Exam Tip: List all five factors—density, pressure, temperature, humidity, and wind—for a complete answer.

Question 7. What is the speed of sound in air at N.T.P?
Answer: \( 330\text{ m/s} \) is the value of speed in air at N.T.P.
In simple words: At normal pressure and temperature, sound travels about 330 meters every single second.

📝 Teacher's Note: N.T.P. stands for Normal Temperature and Pressure. These standard conditions allow scientists to compare results.

🎯 Exam Tip: Memorize the value \( 330\text{ m/s} \) as it is used in almost all numerical problems in this chapter.

Question 8. Compare the speed of sound in water and iron to that in air.
Answer: In water, speed of sound is 4.5 times more than that in air. And, in iron, the speed of sound is 15 times as fast in air.
In simple words: Sound is much faster in water than air, and it's super fast in metal like iron.

📝 Teacher's Note: Sound travels faster when particles are closer together. Particles are closest in solids (iron) and furthest in gases (air).

🎯 Exam Tip: Remember the order: \( \text{Solids} > \text{Liquids} > \text{Gases} \) for the speed of sound.

Question 9. Why does sound travel faster in humid air than in dry air?
Answer: The speed of sound is more in the humid air as compared to dry air. Because, the air moisture decreases the density of air and speed of sound is inversely proportional to density of air. So, sound travels faster in humid air.
In simple words: Humid air is actually "lighter" (less dense) than dry air. Since it's easier for sound to push through lighter air, it moves faster.

📝 Teacher's Note: Many students think humid air is heavier because it feels "heavy," but water vapor is actually less dense than dry nitrogen/oxygen air.

🎯 Exam Tip: The key phrase is "speed of sound is inversely proportional to the density of the medium."

Question 10. Does the speed of sound depend on amplitude or wavelength?
Answer: Speed of sound does not depend on the amplitude and wavelength.
In simple words: Making a sound louder or changing its pitch doesn't make it travel faster.

📝 Teacher's Note: This is a fundamental concept. Wavelength affects the pitch, and amplitude affects loudness, but the *speed* is determined only by the medium.

🎯 Exam Tip: Always remember that speed remains constant in a specific medium at a constant temperature, regardless of loudness.

Question 11. Why do we hear a train's sound through the tracks before hearing it through the air?
Answer: The sound produced by the moving wheels of the train travels faster on the iron track than in the air. So, firstly sound produced because of railway track is heard and then heard in the air.
In simple words: Metal tracks carry sound 15 times faster than air. If you put your ear to the track, the sound "shortcut" reaches you much earlier.

📝 Teacher's Note: Use this as a classic example of why sound speed varies across different states of matter.

🎯 Exam Tip: Mention that iron is a solid and air is a gas, and sound speed is highest in solids.

Question 12. How does sound travel through air?
Answer: The waves in the air causes sound. Through waves, energy get transferred from one place to another and hence the sound.
In simple words: Sound is like a relay race where air particles pass the message of energy to each other until it reaches your ear.

📝 Teacher's Note: Clarify that the air particles themselves don't travel from the source to the ear; only the *energy* travels in the form of a wave.

🎯 Exam Tip: Use the term "energy transfer" to explain the movement of sound waves.

Question 13. What is the ratio of the speed of light to the speed of sound?
Answer: The ratio of speed of light to the speed of sound is \( 9.1 \times 10^{5} \).
In simple words: Light is almost a million times faster than sound!

📝 Teacher's Note: This massive difference is why we see the flash of lightning long before we hear the boom of thunder.

🎯 Exam Tip: You can calculate this by dividing \( 3 \times 10^8\text{ m/s} \) (light) by \( 330\text{ m/s} \) (sound).

Question 14. Is the speed of sound in air affected by changes in pressure?
Answer: Speed of sound in air is independent of variation of pressure.
In simple words: Even if the air pressure goes up or down, sound still travels at the same speed as long as the temperature stays the same.

📝 Teacher's Note: This is a "trick" property. Although pressure changes, the density changes in a way that keeps the ratio (and thus the speed) constant.

🎯 Exam Tip: The keyword here is "independent." Pressure does not change the speed of sound in a gas.

Question 15. How does temperature affect the speed of sound in air?
Answer: The speed of sound in air increases with increase in temperature because with increase in temperature, the density of air gases decreases and speed of air is inversely proportional to density of air.
In simple words: Hot air is less packed together (less dense), which makes it easier for sound to travel faster.

📝 Teacher's Note: On a hot day, sound travels faster than on a cold day. This is a direct result of the "inversely proportional to density" rule.

🎯 Exam Tip: Specifically state: "Temperature increase \( \implies \) Density decrease \( \implies \) Speed increase."

Question 16. Between a person in a boat and a diver underwater, who will hear an explosion first?
Answer: The diver will hear the sound first because the sound travels faster in water than in air.
In simple words: Since water is a liquid, it carries sound much quicker than the air above it.

📝 Teacher's Note: This applies the knowledge that sound is roughly 4.5 times faster in water than in air.

🎯 Exam Tip: Mention the specific medium (liquid vs gas) to justify your answer.

Question 17. How can you calculate the speed of sound using a cliff and a pistol?
Answer: The simple experiment that a person can do to calculate the speed of sound in air is that a person stands at a known distance (\( d \) meter) from the cliff and fires a pistol and simultaneously start the stop watch. He stops the stop-watch as soon as he hears a echo. The distance travelled by the sound during the time (\( t \)) seconds is \( 2d \).
So, \( \text{speed of sound} = \frac{\text{distance travelled}}{\text{time taken}} = \frac{2d}{t} \)
In simple words: Sound has to go to the wall and back, so it travels the distance twice. Divide that double distance by the time it took to hear the bounce.

📝 Teacher's Note: The "2" in \( 2d \) is the most common mistake students make. Remind them that an echo is a "round trip."

🎯 Exam Tip: Always show the formula \( v = \frac{2d}{t} \) when solving echo-based numericals.

Question 18. Summarize the effects of Moisture, Pressure, and Temperature on sound speed.
Answer:

• Moisture in Air: The speed of sound increase with increase in humidity because the moisture in air decrease the density of air and speed of air is inversely proportional to density of air. Hence, the sound travels faster in moist air.
• Pressure in air: Speed of sound is independent of the air pressure.
• Temperature: The speed of sound increases with the increase in temperature of the gas.

In simple words: Heat and humidity make sound go faster. Pressure changes don't do anything to the speed.

📝 Teacher's Note: This summarizes Solution 9, 14, and 15. It's a great "revision block" for the end of the chapter.

🎯 Exam Tip: Examiners often ask for two or three factors that *increase* the speed of sound. Moisture and Temperature are your best answers.

Question 19. If an observer sees a flash and hears the sound after 6 seconds, find the distance (speed = 330 m/s).
Answer: The distance of flash from the observer = \( \text{speed in air} \times \text{time taken for the first sound to reach} \)
\( = 330 \times 6 = 1980\text{m} \)
In simple words: Since sound travels 330 meters every second, in 6 seconds it travels almost 2 kilometers.

📝 Teacher's Note: This problem assumes light travels instantly (it's nearly true for small distances). The calculation is a simple \( D = V \times T \).

🎯 Exam Tip: Always check if the time is for a "one-way" sound (like thunder) or a "two-way" sound (like an echo). Use \( d = vt \) for one-way.

Question 20. Two boys are standing apart. If sound reaches the second boy in 2.5 seconds, find the distance.
Answer: Distance between boys = \( \text{speed of sound} \times \text{time taken to reach sound} = 330 \times 2.5 = 825\text{m} \).
In simple words: If you shout, and your friend hears you 2.5 seconds later, you are about 825 meters away from him.

📝 Teacher's Note: This is a practical application of the basic speed formula. Ensure students use the correct speed value (usually \( 330 \) or \( 340\text{ m/s} \)) as given in the question.

🎯 Exam Tip: Double-check your multiplication with decimals to avoid losing simple marks.