Subjective Type

A tube 1.20 m long is closed at one end. A stretched wire is places near the open end. The wire is 0.330 m long and has a mass of 9.60 g. It is fixed at both ends and oscillates in its fundamental mode. By resonance, it sets the air column in the tube into oscillation at that column's fundamental frequency Find (a) that frequency and (b) the tension in the wire.

Solution

(a) With $$n = 1$$ (for the fundamental mode of vibration) and 343 m/s for the speed of sound, we obtain
$$f = \dfrac{(1) V_{sound}} {4L_{tube}} = \dfrac{343 \space m/s} {4\left (1.20 \space m \right )} = 71.5 \space Hz$$


(b) For the wire we have
$$f' = dfrac{nv_{wire}} {2L_{wire}} = \dfrac{1} {2L_{wire}} \sqrt{\dfrac{\tau} {\mu}}$$
where $$\mu = m_{wire}/L_{wire}$$. Recognizing that

$$f = f'$$ (both the wire and the air in the tube vibrate at the same frequency), we solve this for the tension $$\tau$$.
$$\tau = \left (2L_{wire} f \right )^{2} \left (\dfrac{m_{wire}} {L_{wire}} \right ) = 4f^{2} m_{wire} L_{wire} = 4\left (7105 \space Hz \right )^{2} \left (9.60 \times 10^{-3} \space kg \right ) \left ( 0.330 \space m \right ) = 64.8 \space N$$.


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