Sound – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain why sound needs a medium to travel. Use examples from solids, liquids, gases, and vacuum.
Answer:
- Sound is a form of energy. It travels as vibrations of particles.
- It needs a medium with particles like solid, liquid, or gas.
- In a vacuum, there are no particles. So, sound cannot travel.
- In air, we hear people talk because air particles carry the vibrations.
- In water, sound also moves, so dolphins communicate well.
- In solids, vibrations pass even faster, like through a metal rod.
- In space, you cannot hear anything because there is no medium.
- Thus, a material medium is essential for sound propagation.
Q2. Compare the speed of sound in solids, liquids, and gases with reasons and examples.
Answer:
- Sound travels fastest in solids, then liquids, and slowest in gases.
- In solids, particles are tightly packed. Vibrations pass quickly.
- In liquids, particles are less tight. So, speed is moderate.
- In gases, particles are far apart. The transfer is slow.
- Example speeds: steel ~ 5000 m/s, water ~ 1500 m/s, air ~ 343 m/s.
- Knocking on a wall is heard faster through the wall than through air.
- Under water, sound moves faster than in air, so it reaches sooner.
- So, particle arrangement controls how fast sound moves.
Q3. Why does sound travel fastest in solids? Explain using particle arrangement and energy transfer.
Answer:
- In solids, particles are very close to each other.
- A vibrating particle quickly pushes its neighbor.
- This makes the energy transfer very fast.
- There is less time delay between particle vibrations.
- In liquids and gases, particles are farther apart.
- So, vibrations take longer to pass on.
- Hence, sound is fastest in solids due to tight packing.
Q4. Describe an activity to show sound travels faster in solids than in air. State observations and conclusion.
Answer:
- Take a metal rod. Tap one end with a spoon.
- Ask a friend to put an ear on the other end.
- Repeat with a wooden stick and a plastic straw.
- Also listen through air without touching the rod.
- Observation: The sound through the solid rod reaches earlier and is clearer.
- The rod carries vibrations faster than air.
- Conclusion: Sound travels faster in solids than in gases like air.
Q5. What is an echo? How can you use echo to estimate the speed of sound? Explain with steps.
Answer:
- An echo is the reflection of sound from a distant surface.
- You clap or shout. The sound hits a wall and comes back.
- Measure the time between the sound and the echo.
- Total distance the sound travels is 2 × distance to the wall.
- Speed formula: Speed = Distance / Time.
- So, speed of sound = (2 × distance to wall) / echo time.
- Use air speed near 343 m/s at room temperature to check your result.
- This method uses reflection to find speed.
High Complexity (Analysis & Scenario-Based)
Q6. During a thunderstorm, you see lightning first and hear thunder later. Explain why. Also estimate distance to the storm using timing.
Answer:
- Light travels much faster than sound.
- So, you see the lightning almost instantly.
- The sound of thunder takes time to reach you.
- That is why there is a delay.
- To estimate distance: distance ≈ speed of sound × time delay.
- Take speed in air ≈ 343 m/s. If delay is 5 s, distance ≈ 343 × 5 ≈ 1715 m.
- So, the storm is about 1.7 km away.
- This method uses speed, time, and simple multiplication.
Q7. In a bell jar with a ringing alarm, air is slowly removed by a vacuum pump. Predict and explain what you will observe.
Answer:
- At first, the alarm is loud because air is present.
- As air is removed, the loudness goes down.
- Fewer particles are left to carry vibrations.
- Near vacuum, the sound becomes very faint or inaudible.
- The alarm still vibrates, but there is no medium to carry sound.
- This shows sound needs a material medium.
- In vacuum, sound cannot travel.
Q8. Two swimmers are fully underwater and trying to talk. Will the sound be clearer than in air? Explain using particle spacing and speed.
Answer:
- Under water, particles are closer than in air.
- So, sound moves faster and with less loss.
- Swimmers underwater can hear clearer than in air at similar distances.
- The energy passes quickly from one water particle to another.
- That is why marine animals communicate well in water.
- But the direction of sound may feel different underwater.
- Overall, sound is clearer and quicker in liquids than in gases.
Q9. A student puts an ear on a steel rail and another ear in air near the rail. A friend taps the rail far away. Which sound arrives first and why?
Answer:
- The sound through steel arrives first.
- Steel is a solid with tightly packed particles.
- Speed in steel is about 5000 m/s.
- In air, speed is about 343 m/s.
- So, vibrations in steel reach the ear much earlier.
- The air-borne sound arrives later due to lower speed.
- This proves sound is fastest in solids.
Q10. You clap in a long hallway and hear an echo after 0.8 s. Estimate the distance to the wall. Explain each step.
Answer:
- The clap goes to the wall and comes back.
- Total travel time = 0.8 s for the round trip.
- Time to reach the wall = 0.8 / 2 = 0.4 s.
- Use speed of sound in air ≈ 343 m/s.
- Distance to wall = speed × time one-way.
- So, distance ≈ 343 × 0.4 ≈ 137.2 m.
- The wall is about 137 m away.
- This uses echo, time halving, and Speed = Distance / Time.