A rod of mass 'M' and length '2L' is suspended at its middle by a wire. It exhibits torsional oscillations; if two masses each of 'm' are attached at distance 'L/2' from its centre of both sides, it reduces the oscillation frequency by 10%. The value of the ratio M/m is close to :

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JSSC JE Mechanical 3 July 2022 Official Paper - II
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  1. 6.4
  2. 7.7
  3. 3.7
  4. None of these

Answer (Detailed Solution Below)

Option 1 : 6.4
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Detailed Solution

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Concept:

The frequency of a Torsional Pendulum is given by

where K = Torsional Constant, I = Moment of Inertia 

Calculation:

Given:

Mass of the rod = M, Length of the rod = 2L, Mass of 2 attached masses = m, Location of attachment = L/2 from center of rod of both sides,

Reduction in frequency = 10 % 

⇒ 

⇒ f2 = 0.9f1   ................. (i)

Case 1:

Moment of Inertia, 

Frequency of Oscillation, 

Case 2:

Moment of Inertia, 

Frequency of Oscillation, 

From equation (i), we have

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