A2 Daily A Level Physics question
In a resonance-tube experiment, a vertical tube of length 0.50 m is closed at the bottom and open at the top. The lowest-frequency resonance with a tuning fork occurs at 170 Hz. Assuming uniform sound speed in air, what is the approximate speed of sound?
Answer
The correct answer is C.
Correct: C — 340 m/s. A closed–open tube has its lowest mode when the length is a quarter of a wavelength, so λ = 4L = 2.0 m and v = fλ = 170 × 2.0 ≈ 340 m/s. A 85 m/s results from wrongly taking λ = L as if the tube supported a full wavelength at the lowest mode. B 170 m/s comes from assuming λ = 2L (both ends open or a stretched string) rather than a quarter-wave. C is correct because the closed end is a displacement node and the open end an antinode, giving a quarter-wave fundamental. D 680 m/s would require λ = 8L (or misidentifying the observed resonance as a higher odd harmonic).