NCERT 8th Class (CBSE) Science: Sound

Question: Ramesh measured the time gap between lightning and thunder and found that it was 3 seconds. Assuming light cover the distance instantly, what is the distance of cloud from Ramesh?

Answer: Time gap = 3 sec

Speed = 340 m/s

Speed = Distance/Time

Therefore, Distance = Speed * Time

Distance = 340 * 3= 1020 m = 1 km 20 sec.

The distance of clouds from Ramesh is 1 km 20 sec.

Question: Why can you not hear the echo if you stand 1 0 m away from a wall and shout towards it?

Answer: To hear the echo we need to stand at least 17 m away from the object. This is because, distance travelled by sound in 1/10 second = speed x time = 340 m/s x (1/10) s = 34 m. Thus, we cannot hear the echo if you stand 10 m away from a wall and shout towards it.

Speed of sound in air = 340 m/s

Minimum time taken for human to hear echo = 0.05 sec

Since, S = V * T (S= distance; V= Speed/velocity; T=Time)

S= 340 * 0.05 = 17 m

Therefore, the minimum distance to hear an echo is 17 m.

Question: How is sound produce by our throat?

Answer: Humans produce sound in the larynx due to vibrations in the vocal cords. Vocal cords are folds of tissues with a slit-like opening between them. When we speak, air passes through the opening and the vocal cords vibrate to produce sound.

Question: Name the three type of musical instrument . How is sound produced by each?

Answer: There are three types of musical instruments. They are:

1. Stringed instruments: Sitar, guitar, violin, Sarod, etc., are stringed instruments. They use vibrations in metal wires to produce sound. The frequency of sound is varied by varying the length of the vibrating wire.
2. Wind instruments: Flute, shehnai, etc., are wind instruments. They use vibrations in air columns to produce sound.
3. Percussion instruments: Drum, tabla, Mridangam, etc., are percussion instruments. They have a stretched skin whose vibrations produce sound. Loudness is increased by striking the skin harder. The pitch is increased by stretching the skin more.

Question: What do you mean by noise pollution? What are the main causes of noise pollution?

Answer: Too much noise in our surroundings produced by Irregular vibrations is harmful to us and is referred to as noise pollution. The main causes of noise pollution are:

1. Horn blown by Traffic on the road,
2. Noise produced by machines in factories
3. In our hose noise is cause by mixers and grinders, vacuum cleaners, air coolers, etc
4. Loud music produced by speaker