NCERT Solutions For Class 9 Science

How does the sound produced by a vibrating object in a medium reach your ear?

Explain how sound is produced by your school bell.

Why are sound waves called mechanical waves?

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Which wave property determines (a) loudness, (b) pitch?

Guess which sound has a higher pitch: guitar or car horn?

What are wavelength, frequency, time period and amplitude of a sound wave?

How are the wavelength and frequency of a sound wave related to its speed?

Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Distinguish between loudness and intensity of sound.

In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s–1?

Why are the ceilings of concert halls curved?

What is the audible range of the average human ear?

What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?

What is sound and how is it produced?

Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Why is sound wave called a longitudinal wave?

Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s⁻¹.

Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Does sound follow the same laws of reflection as light does? Explain.

When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?

Give two practical applications of reflection of sound waves.

A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s⁻² and speed of sound = 340 m s⁻¹.

A sound wave travels at a speed of 339 m s⁻¹. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

What is reverberation? How can it be reduced?

What is loudness of sound? What factors does it depend on?

How is ultrasound used for cleaning?

Explain how defects in a metal block can be detected using ultrasound.