CBSE Class 9 Science Chapter 12 Sound NCERT Solutions
CBSE Class 9 Science Chapter 12, focusing on Sound, introduces students to the fascinating world of acoustics. This chapter explains that sound originates from vibrating objects and travels as waves through a medium. The NCERT Solutions clarify that sound waves are mechanical waves, meaning they need a substance like air, water, or solids to move, unlike light waves. You'll learn about compressions and rarefactions, which are the high and low-pressure regions that make up a sound wave. The solutions also address why sound cannot travel through a vacuum, using examples like space. These explanations are designed to build a solid understanding of sound's properties and its transmission, crucial for both exams and a deeper grasp of physics.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 9 |
| Subject | Science |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 12 |
Chapter summary
Chapter 12, 'Sound,' for Class 9 Science NCERT Solutions focuses on the production and propagation of sound. It explains that sound is generated by vibrating bodies and travels as mechanical waves through compressions and rarefactions in a medium. The solutions clarify why sound requires a medium and cannot travel through a vacuum, using examples like the moon. This chapter is crucial for understanding the basic physics of sound.
Learning outcomes
- Understand how sound is produced by vibrating objects.
- Explain the concept of sound waves as mechanical waves.
- Describe the process of sound propagation through compressions and rarefactions.
- Explain why sound cannot travel in a vacuum.
- Relate the absence of sound on the moon to the lack of a medium.
Topics covered
Paper topics
- Sound Production
- Vibrating Objects
- Sound Waves
- Mechanical Waves
- Medium for Sound
- Compression
- Rarefaction
- Sound Propagation
- Sound in Vacuum
- Sound on the Moon
Important topics
- Sound Production by Vibrations
- Mechanical Nature of Sound Waves
- Role of Medium in Sound Propagation
- Sound Cannot Travel in Vacuum
PDF preview
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Questions and Solutions
Question 1
When an object vibrates, it disturbs the particles of the surrounding medium (like air). These vibrations cause the particles near the vibrating object to be pushed together, creating a region of higher density and pressure called a compression. Immediately after, as the object moves back, the particles spread apart, creating a region of lower density and pressure called a rarefaction. These compressions and rarefactions travel outwards from the source as a wave. As the wave passes through the medium, the particles of the medium vibrate back and forth about their mean positions, transferring energy. This chain of compressions and rarefactions propagating through the medium eventually reaches your ear, where it causes your eardrum to vibrate, which is interpreted by your brain as sound.
Question 1
When you strike a school bell with a hammer or clapper, the bell itself begins to vibrate. These vibrations are rapid back-and-forth movements of the bell's material. As the bell vibrates, it pushes and pulls on the surrounding air particles. This action creates compressions (regions of high pressure) and rarefactions (regions of low pressure) in the air, which propagate outwards as sound waves. When these sound waves reach your ears, they cause your eardrum to vibrate, and your brain interprets these vibrations as the sound of the bell.
Question 2
Sound waves are called mechanical waves because they require a material medium to travel from one place to another. Unlike electromagnetic waves (like light), which can travel through a vacuum, sound waves need particles (like those in air, water, or solids) to vibrate and transmit the energy. If there is no medium, such as in a vacuum, sound cannot propagate.
Question 3
No, you will not be able to hear any sound produced by your friend on the moon. This is because the moon has virtually no atmosphere, meaning there is no medium (like air) for sound waves to travel through. Sound waves are mechanical waves and require a medium to propagate. Since there is a vacuum on the moon, any sound produced will not be transmitted to your ears.
Common mistakes
- Confusing mechanical waves with electromagnetic waves.
- Assuming sound can travel through a vacuum.
- Not clearly explaining the role of vibrations in sound production.
Revision tips
- Focus on the definition and examples of mechanical waves.
- Clearly differentiate between compression and rarefaction.
- Understand the necessity of a medium for sound travel.
- Use the moon example to reinforce the concept of sound in a vacuum.
Practice MCQs
Q1. What causes sound to be produced by a vibrating object?
Explanation: Sound is produced when an object vibrates, causing disturbances in the surrounding medium.
Q2. Why are sound waves classified as mechanical waves?
Explanation: Mechanical waves, like sound waves, need a medium such as air, water, or solids to travel from one point to another.
Q3. What are the two regions of alternating high and low pressure that constitute a sound wave?
Explanation: Sound waves travel through a medium by creating regions of compression (high pressure) and rarefaction (low pressure).
Q4. Can sound be heard on the moon?
Explanation: Sound requires a medium to travel. The moon has no atmosphere, meaning there is no medium for sound to propagate.
Frequently asked questions
What is sound?
Sound is a form of energy that is produced by vibrating objects and travels through a medium as waves, reaching our ears and allowing us to hear.
How is sound produced?
Sound is produced when an object vibrates. These vibrations disturb the particles of the medium around the object, creating sound waves.
What are mechanical waves?
Mechanical waves are waves that require a material medium (like air, water, or solids) to propagate. Sound waves are an example of mechanical waves.
Can sound travel through space or a vacuum?
No, sound cannot travel through a vacuum because there are no particles to vibrate and carry the sound energy. This is why you cannot hear sounds on the moon.
What are compressions and rarefactions in sound waves?
Compressions are regions in a sound wave where particles are crowded together (high pressure), and rarefactions are regions where particles are spread apart (low pressure).
Why is it important to have a medium for sound to travel?
Sound waves are mechanical waves, meaning they transfer energy by causing particles in a medium to vibrate. Without particles (a medium), there is nothing to vibrate, and thus no sound can travel.
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