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1 Physics -- Mechanical Waves

In a stormy day a boy observes a lightening flash which is followed by a thunder 3 sec. later. How would you estimate the distance of the lightening strike from the boy? ( given, velocity of sound on that day= 332 m/s, velocity of light c= 3*10^8 m/s.

In a stormy day a boy observes a lightening flash which is followed by a thunder 3 sec. later. How would you estimate the distance of the lightening strike from the boy? ( given, velocity of sound on that day= 332 m/s, velocity of light c= 3*10^8 m/s.

2 Answers

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Time taken (t) = 3 sec
Velocity of sound (v) = 332 m/s
Distance travelled (d) = ?

We know,

d = v x t = 332 x 3 = 996 m

Hence, the source of thunder is about 996 m far from the boy.

In this question, we have nothing to do with the velocity of light. The basic concept lies in the velocity of light is way greater than the velocity of sound. So, the light reaches the ground earlier than the sound reaches the ground.

Here, it says followed by. So, the time must have been calculated after the lightning was observed. Therefore, the actual distance between the origin of lightning and the observer is obtained by the velocity of sound.

Hence

distance (d) = velocity of sound * time

= 332 * 3

= 996 m

More questions on Mechanical Waves

The speed of light does not depend upon the temperature of air but the speed of sound depends on it. Why?

As we know that wave of sound is a mechanical wave and its speed is given by

v =(γRT/M)

whereas the wave of light is an electromagnetic waveand its speed is given by

c =(1/εoμo)

So speed of sounddepends on temperature of air but speed of light doesn't.

What is the minimum distance between a source of the sound and a reflector to hear an echo?

Let S be the distance between the source of sound and reflector.
V be the velocityof sound in air.
t be the total time taken by the sound to travel from source to reflectorand back to the source.

2S = Vt
or,S = (Vt)/2

We can hear distinct sounds if they reach our ear after an interval of 0.1s or more
That is, t = 0.1s and V = 340 ms-1

Therefore, S = (340 x 0.1)/2 = 17 m.
This is required minimum distance to hear an echo.

During a lightning, light and sound are produced simultaneously but we see light before we hear the sound. Why?

It is so because the speed of light (3 x 108 m/s) is much greater than the speed ofsound (332 m/s) in a gas. Thus, light reaches immediately and sound reaches later tothe earth's surface.

A source of sound produces a note of the frequency of 512 Hz in the air at 17°C with wavelength 66.5 cm. find the ratio of molar heat capacities at constant pressure to constant volume at NTP. Dens...

The speed of sound in moist air is greater than that in dry air, why? Will the speed of sound in moist hydrogen be greater than that in dry hydrogen?

The lower the density, the faster sound travels in a medium. The density of water vapour is less than that of air. So the density of air mixed with water vapour is less than that of dry air. Hence the speed of sound in moist air is greater than that in dry air(v1/ρ) . However, the density of water vapour is more than that of hydrogen, so the density of moist hydrogen is more than that of dry hydrogen. Consequently, the speed of sound in moist hydrogen is less than that in dry hydrogen.

The speed of sound in moist air is greater than that in dry air, why? Will the speed of sound in moist hydrogen be greater than that in dry hydrogen?

The lower the density, the faster sound travels in a medium. The density of water vapour is less than that of air. So the density of air mixed with water vapour is less than that of dry air. Hence the speed of sound in moist air is greater than that in dry air(v1/ρ) . However, the density of water vapour is more than that of hydrogen, so the density of moist hydrogen is more than that of dry hydrogen. Consequently, the speed of sound in moist hydrogen is less than that in dry hydrogen.

What is the drawback of Newton's formula for the velocity of sound in gas? What do you understand by Laplace's correction? Discuss the effect of temperature and pressure on velocity of sound.

The drawback of Newton's formula for velocity of sound are:
1. The velocity of sound calculated from Newton's formula is279.9 m/s at N.T.P. Experimentally, they found that the velocity of sound in air at N. T. P is 332 m/s. Thus, there is a considerable difference between the value predicted by Newton’s formula and the experimental value.

2. Experimental value is 16% greater than the value given by the formula. Newton failed to satisfactorily explain the difference.


Laplace Correction:

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1. Solids: Sound travels fastest in solids. The velocity of sound in solids ranges from approximately 2000 m/s to 8000 m/s.

2. Liquids: Sound travels slower in liquids compared to solids. The velocity of sound in liquids is approximately 1000 m/s to 1500 m/s.

3. Gases: Sound travels slowest in gases. The velocity of sound in gases is around 343 m/s at room temperature.

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