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Determine unknown frequency with the help of beats

Two frequencies sounded together produce 3 beats per second.If one of the frequencies is 400 vibrations per second,the other frequency will be?


Asked Tayyab Mynir




Beating Frequency


There are two possibilities because the beat frequency is equal to the difference in frequencies of the two sounding bodies.


So, the unknown frequency can be either 397 Hz or 403 Hz. This can be verified by lagging/ loading one of the sounding bodies.


Read more at http://en.wikipedia.org/wiki/Beat_(acoustics)


Calculating velocity of an approaching vehicle

Is it possible to design a device that calculates the velocity of an approaching vehicle if the speed of sound in air at that location  is known?


Asked Vibhava




Now the source is moving at the speed of sound...


Such a device is already in use. It is based on a phenomenon called Doppler Effect.


Get the details at the following Links.


  1. www.physicsclassroom.com/class/waves/u10l3d.cfm
  2. science.howstuffworks.com/science-vs-myth/everyday-myths/doppler-effect3.htm
  3. http://en.wikipedia.org/wiki/Doppler_effect



Questions from Class XI Physics

1. two boats moving parallel to each other and nearby are pulled towards each other. name the scientific theorem involved in the above cases. state and prove the theorem.

2. what are beats?name the basic phenomenon due to which beats are produced.two sources of sound are producing waves of frequency v1 and v2 where ( v1-v2) is small. show mathematically that the beat frequency is (v1-v2).


Beating of two frequencies

3. a body is projected with a velocity ‘u’ at angle theta with the horizontal. derive the mathematical equation of the path followed by the body and hence find the expression for the horizontal range. how will the horizontal range change if the angle of projection is changed to (90- theta).

4. using expression for pressure exerted by a gas on the walls of container, deduce Avogadro’s law.

5. a monkey of mass 40kg climbs on a rope which can with stand a maximum tension of 600N. in which of the following cases will the rope break:

a) if the monkey climbs up with an acceleration of 6m/s2
b) if the monkey climbs down with an acceleration of 4m/s2
c) if the monkey climbs up with a uniform speed of 5m/s
( ignore mass of rope. take g= 10 m/s2)

6. what is the need of banking of tracks?

7. the absolute temperature of a gas is increased to four times. what will be the change in its root mean square speed?

8. find the expression for apparent frequency observed by a stationary observer when a sound emitting source is approaching towards the observer.

9. a body weighs 63N on the surface of earth. what is the gravitational force on it due to the earth at a height equal to half the radius of the earth.

10. show that the co- efficient of superficial expansion of a rectangular sheet of the solid is twice its co-efficient of linear expansion.




(Posted by Ritika)


(Post your answers now!



How light maintain its speed after refraction.

Dr. Naveen Saxena

n=c/v,naturally velocity of light decrease in refraction.but even even then it has the velocity in the order of 108m.In refraction,

the frequency of light remains the same ,but the wavelength




Wave travelling from one medium to another

Iqbal Lucky asked

“when a wave enters from air to water, its wavelength changes but

frequency remains unchanged. why”


Velocity of a wave = frequency x time period

V = ν λ

When a wave travels from one medium to another, its velocity changes and so also is its wavelength, but frequency; which is the number of vibrations produced per second is independent of the medium, remains constant.

However, their is a phenomenon called Doppler Effect where the apparent frequency (observed frequency) is different from actual frequency and is caused by the relative movement between the source and observer (listener)

Got a better answer?

Please post it as comment.

Factors affecting frequency of sound produced by a stretched string

Smile asks:

Study how the frequency of sound produced will change in each case with the following strings of length 15cms when the strings are tied between  2 ends-

  • aluminium string
  • copper string
  • cotton string
  • metallic string
  • jute string

Also study how the pitch changes when the strings are made taught and loose. Study how the frequency of sound changes with thickness of the following strings

  • cotton strings
  • copper strings


This seems to be a homework question or a project question. Therefore I am not giving a detailed answer so as not to tamper the basic aim of assigning a project.

The frequency of sound produced by a stretched string depends on the following factors:

  1. the length of the string
  2. the linear mass density (i.e; the mass per unit length) of the string
  3. the tension in the string

When you are using strings of different materials, the factor which changes is the mass per unit length and the same is true when you are changing the thickness.

When you make the string more taut, the tension increases and vice versa.

The question is given for a constant length. Therefore the case of effect of changing length does not come into picture.

The formula showing the relationship is 

it is evident from the formula that the frequency of sound is

  • inversely proportional to the  length
  • directly proportional to the square root of tension in the string and
  • inversely proportional to the square root of linear density of the string.

on proper substitution, the formula can be recast as

and this will be more convenient for you to answer the questions.

I recommend that you try to explore by actually performing the experiments.

Refer to http://www.tutorvista.com/content/physics/physics-iii/waves/stretched-strings-vibration.php for more

Doppler Effect

Hasan asks:

“What is Doppler Effect?”



Doppler Effect is the apparent change in frequency of a wave due to the relative motion between the source and the listener.

(Please come back soon for more details with figures and videos)

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