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

Question

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

vincent-lyon.fr · Answer

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.

vincent-lyon.fr · Answer

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

ujjwal · Answer

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..

ujjwal · Answer

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

jamesj · Answer

@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.

asnaseer · Answer

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

asnaseer · Answer

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

asnaseer · Answer

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

asnaseer · Answer

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

ujjwal · Answer

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

asnaseer · Answer

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

ujjwal · Answer

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

asnaseer · Answer

think of the sinusoidal wave generated as each holes passes by

asnaseer · Answer

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

asnaseer · Answer

maybe someone else can give a better explanation?

asnaseer · Answer

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

ujjwal · Answer

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