CBSE Class 8 Science Chapter 13 Sound NCERT Solutions

Solution: Sound is a form of energy that is produced by vibrations. These vibrations travel through a medium. Sound can propagate through solids, liquids, and gases because all these states of matter consist of particles that can vibrate and transmit the sound energy. Therefore, the correct option is (d).

Solution: The frequency of a sound is related to its pitch. A lower frequency corresponds to a lower pitch, while a higher frequency corresponds to a higher pitch (shrillness). Adult men typically have larger and thicker vocal cords compared to women and children. This results in slower vibrations of the vocal cords, producing a sound with a lower frequency and thus a lower pitch. Therefore, a man's voice is likely to have the minimum frequency among the given options.

Solution:

1. Sound cannot travel in vacuum. (T) Sound requires a medium to travel. In a vacuum, there are no particles to vibrate, so sound cannot propagate.
2. The number of oscillations per second of a vibrating object is called its time period. (F) The number of oscillations per second is called frequency. The time period is the time taken to complete one oscillation.
3. If the amplitude of vibration is large, sound is feeble. (F) The loudness of a sound is determined by the amplitude of vibration. A large amplitude results in a loud sound, not a feeble one.
4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T) This is the generally accepted range of frequencies that the average human ear can detect.
5. The lower the frequency of vibration, the higher is the pitch. (F) The pitch of a sound is directly proportional to its frequency. A lower frequency results in a lower pitch, while a higher frequency results in a higher pitch (shrillness).
6. Unwanted or unpleasant sound is termed as music. (F) Unwanted or unpleasant sound is termed as noise. Music is generally considered pleasant sound.
7. Noise pollution may cause partial hearing impairment. (T) Prolonged exposure to loud noise (noise pollution) can damage the sensitive structures in the ear, leading to hearing loss.

Solution:

(a) Time taken by an object to complete one oscillation is called time period.

(b) Loudness is determined by the amplitude of vibration.

(c) The unit of frequency is hertz (Hz).

(d) Unwanted sound is called noise.

(e) Shrillness of a sound is determined by the frequency of vibration.

Solution:

To find the time period and frequency of the pendulum, we use the given information:

Number of oscillations = 40

Time taken = 4 seconds

The time period is the time taken for one complete oscillation. It can be calculated as:

$$Time\backslash \; period\; =\; \backslash frac\{Time\backslash \; taken\}\{Number\backslash \; of\backslash \; oscillations\}$$

$$Time\backslash \; period\; =\; \backslash frac\{4\backslash \; seconds\}\{40\}\; =\; 0.1\backslash \; seconds$$

The frequency is the number of oscillations completed per second. It can be calculated as:

$$Frequency\; =\; \backslash frac\{Number\backslash \; of\backslash \; oscillations\}\{Time\backslash \; taken\}$$

$$Frequency\; =\; \backslash frac\{40\}\{4\backslash \; seconds\}\; =\; 10\backslash \; Hz$$

Therefore, the time period of the pendulum is 0.1 seconds and its frequency is 10 Hz.