A circular disk has 100 holes and it is rotating at the rate of 720 cycle/min..the frequency of emitted note will be a)1200 Hz b)70 Hz c)8400 Hz d)7.2 Hz

I imagine a jet of compressed air is blown where the holes are? If not, I can't see how such a system could produce a sound wave.

Work out how many times per second the air blows through a hole, and this is your frequency.

I guess the question says the air is blown 720 times per min, i.e 120 times per sec, through all 100 holes... I don't know how do solve this one..

@pastory i am sure you gave the answer by simply guessing.. But that's not in the list of options...

@pastory, no that isn't right. Assuming the 100 holes are all at the edge, then in one minute, 720x100 = 72,000 holes are blown per minute or 72,000/60 = 12,000 per second.

720 cycles per minute, implies: 1 cycle in \(\frac{1}{720}\) minutes = \(\frac{1}{12}\)s therefore 360 degrees in \(\frac{1}{12}\)s therefore 1 degree in \(\frac{1}{12*360}\)s 100 holes would take up \(\frac{360}{100}\) degrees = 3.6 degrees therefore 3.6 degrees in \(\frac{3.6}{12*360}\)s = \(\frac{1}{1200}\)s and your frequency would be this fraction inverted, i.e. 1200Hz

JamesJ - I /think/ you forgot to cancel one zero

@ujjwal I used the fact that frequency \(f=\frac{1}{T}\)

sorry - in my explanation above I should have said "1 hole would take up..." instead of "100 holes would take up..."

100 holes would take up 360/100 degrees = 3.6 degrees I didn't get this one..

100 holes take up 360 degrees therefore each hole would be separated by \(\frac{360}{100}\) degrees

And that is frequency for rotation of each hole.. How does it end up as the frequency of note emitted?

think of the sinusoidal wave generated as each holes passes by

so you get a "peak" flow when the hole is in line with the air being blown, and it decays as the hole spins past

maybe someone else can give a better explanation?

imagine flicking your finger across your mouth as you blow out - you generate a sound wave whose frequency is equal to the frequency of your finger flicking

|dw:1338134955778:dw| which way is the disk rotating?

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