## Q1: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Ans.
Given that,
Number of oscillation    = 40

Total time taken        = 4 seconds

Time period                = time taken in one oscillation

= $$\frac{Total time}{Total number of oscillations}$$

= $$\frac{4 seconds}{40}$$

= $$\frac{1}{10}$$ = 0.1 second

Again, frequency       = number of oscillations per second

= $$\frac{Number of vibrations }{ Time taken}$$

= $$\frac{40}{4}$$

= 10 per second = 10 Hz.

## Q2: The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Ans.

Number of vibrations per second = 500

Time period = time taken for one vibration

= $$\frac{Total time}{Total number of vibrations}$$
= $$\frac{1}{500}$$ Hz
= 0.002 Hz.

## (a) Dholak            (b) Sitar            (c) Flute

Ans.

(a) Stretched membrane

(b) String of sitar

(c) Air column.

## Q4: What do you understand by stringed instruments?

Ans. Those instruments have taut strings, which vibrate when they are plucked, struck, or played with a bow.

## (v) The sound of a bursting balloon.

Ans.

(i) Strings of sitar or veena vibrate to produce sound.

(ii) The skin of the tabla vibrate to produce sound.

(iii) The disc of the bell when beated with hammer vibrate to produce sound.

(iv) Wings of the mosquito or a bee vibrate to produce sound.

(v) Air vibrates to produce sound.

(vi) In the radio speakers vibrate to produce sound.

## Q6: What is the normal length of vocal cords in man?

Ans. The normal length of the vocal cord of a man is about 20 mm long.

## Q7: What is the unit of the loudness of sound? Explain giving the loudness of few sounds.

Ans. The loudness is expressed in a unit called decibel (dB). It is essentially a ratio if the actual loudness to some base loudness level. The base level is usually defined as that loudness of sound that the human ear can just perceive. The following table gives some idea of the loudness of sound coming from various sources:

At 80 dB the noise becomes physically painful.

## Q8: We hear sound only if its frequency is greater than ……. and lower than ……. .

Ans. 20 hertz, 20000 hertz.

Ans. Higher.