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Can light do some mechanical work?

“We know that sound can cause some mechanical work(eg:when we keep our hand in front of a large speaker,we feel something hitting the back of our hand), but why this doesn’t happen with light??” -Manishankar asks.

Relativity, Black Hole and some doubts

In the model of the big bang I would like to pose the following question. My understanding is that

1. The expansion of the universe is accelerating and

2. The radiation or shock wave to keep the analogy going is something we can measure.

My question is this. While we can rewind the clock to a single moment when the universe was infinity dense how do we know that this was a moment at all. Let me elaborate. As the universe expands could time or space time not also expand and if so than when the big bang occurred is something we cannot measure merely by rewinding the clock. If we are experiencing time at a given rate when the universe is at its current size would time not be being effected by the expansion also and if so would that mean the there was simply no start and no end to the universe? Perhaps the big bang is not as accurate as we may wish and perhaps it is our ability to comprehend that is limited. I propose there is no start or end to the universe it has always and will always be as it, and we, cannot exist without time and as such these two objects or things form a symbiotic relationship with each other. While the universe expands so does time and its effects are reduced in line with its expansion. Does this sound plausible and if not why?

Question from Beats (Waves)

Sanjeet asked:

"the displacement of a particle in periodic motion is given by "y=(cos2t-2cost+2sin^2t+1). What is the no. of beats that can be heard in 10 seconds?"

Working of a microphone

How does a microphone do its work? Please explain me and can you give me answer in language Hindi I’m very thankful to you. (Shubham Agnihotri asked)

inside a microphone
Microphone is a transducer (a device which converts energy from one form to another). The microphone converts sound energy to mechanical energy and then to electrical energy.

When we speak into the mic, the vibrations of the air produced by the sound make the diaphragm of the mic vibrate. A coil attached to the diaphragm vibrates together with it. The coil is moving in a strong magnetic field produced by a permanent magnet. Whenever the magnetic flux linked with coil changes an emf is induced in it. Therefor when we speak into the mic, an emf varying similar to the pressure variations produced by our sound is produced across the coil by electromagnetic induction.

If the ends of the coil is connected to an amplifier this can be amplified and can be send to the loudspeaker. The working of the loudspeaker is just the reverse of that of the microphone. I converts the electrical signal to sound signals.

(Please use Google Translate  or Click Here to get the Hindi Version)

Why a sound is produced when a fluorescent tube breaks?

Why does a big  sound come when tube-light breaks?

(Question was posted by Vishwesh)

Answer: Every sound is produced by some vibrating bodies. In this case it is air surrounding the tube which vibrates.

Broken Tube LightInside the tube-light there is partial vacuum. When it breaks, the air surrounding the tube suddenly rushes into the vacuum and starts vibrating and thus produces the sound.

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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