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A source of unknown frequency gives 4 beats/s, when sounded with a source of known frequency 250 Hz. The second harmonic of the source of unknown frequency gives five beats per second, when sounded with a source of frequency 513 Hz. The unknown frequency is [NEET 2013]
  • a)
    246 Hz
  • b)
    240 Hz
  • c)
    260 Hz
  • d)
    254 Hz
Correct answer is option 'D'. Can you explain this answer?
Verified Answer
A source of unknown frequency gives 4 beats/s, when sounded with a sou...
When sounded with a source of known frequency fundamental frequency = 250 ± 4 Hz = 254 Hz or 246 Hz
2nd harmonic if unknown frequency (suppose) 254 Hz = 2 × 254 = 508 Hz
As it gives 5 beats
∴  508 + 5 = 513 Hz
Hence, unknown frequency is 254 Hz
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Given data:
- When the unknown frequency source is sounded with a known frequency source of 250 Hz, it produces 4 beats per second.
- When the second harmonic of the unknown frequency source is sounded with a known frequency source of 513 Hz, it produces 5 beats per second.

Let's analyze the given information and find the unknown frequency step by step.

Step 1: Finding the unknown frequency using the first harmonic
- When two sources with different frequencies produce beats, the frequency of the beats is equal to the difference in frequencies between the two sources.
- In this case, when the unknown frequency source is sounded with a known frequency source of 250 Hz, it produces 4 beats per second.
- Thus, the difference between the unknown frequency and 250 Hz is 4 Hz.
- Therefore, the unknown frequency can be written as f1 = 250 Hz ± 4 Hz.

Step 2: Finding the unknown frequency using the second harmonic
- The second harmonic of a frequency is twice the frequency itself.
- When the second harmonic of the unknown frequency source is sounded with a known frequency source of 513 Hz, it produces 5 beats per second.
- Thus, the difference between the second harmonic of the unknown frequency and 513 Hz is 5 Hz.
- Therefore, the second harmonic of the unknown frequency can be written as 2f1 = 513 Hz ± 5 Hz.

Step 3: Solving the equations to find the unknown frequency
- From step 1, we have f1 = 250 Hz ± 4 Hz.
- From step 2, we have 2f1 = 513 Hz ± 5 Hz.
- By comparing the two equations, we can see that the unknown frequency f1 is the same in both equations.
- Solving the equations simultaneously, we can eliminate the ± signs and solve for the unknown frequency f1.
- Subtracting the first equation from the second equation, we get: 2f1 - f1 = 513 Hz - 250 Hz ± (5 Hz - 4 Hz).
- Simplifying the equation, we get: f1 = 263 Hz ± 1 Hz.

Step 4: Final answer
- From step 3, we found that the unknown frequency f1 is 263 Hz ± 1 Hz.
- Among the given options, the only option that falls within this range is 254 Hz (263 Hz - 1 Hz = 262 Hz, 263 Hz + 1 Hz = 264 Hz).
- Therefore, the correct answer is option 'D' - 254 Hz.
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A source of unknown frequency gives 4 beats/s, when sounded with a source of known frequency 250 Hz. The second harmonic of the source of unknown frequency gives five beats per second, when sounded with a source of frequency 513 Hz. The unknown frequency is [NEET 2013]a)246 Hzb)240 Hzc)260 Hzd)254 HzCorrect answer is option 'D'. Can you explain this answer?
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