Q 11-14-004NEETNEET 2020Top questionMedium
In a guitar, two strings A and B made of same material are slightly out of tune and produce beats of frequency $6$ Hz. When tension in B is slightly decreased, the beat frequency increases to $7$ Hz. If the frequency of A is $530$ Hz, the original frequency of B will be :
Answer: (B) $524$ Hz
Concept. Beat frequency $= |f_A - f_B|$. Lowering the tension lowers $f_B$ ($f \propto \sqrt T$); see which choice makes the beats increase.
Step 1. Possible values. $f_B = 530 \pm 6 = 524$ or $536$ Hz.
Step 2. Test.
- $536$ Hz: lowering it moves it towards $530$, so beats would decrease. ✗
- $524$ Hz: lowering it moves it away from $530$, so beats increase to $7$ Hz. ✓
Answer. $524$ Hz (Option B).
Solution by Sreeraj P, M.Sc Physics