Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 12.14
[10 points.] The source and observer of a sound wave are both at rest with respect to the ground. The wind blows in the direction from the source to observer. Discuss whether the observed frequency is higher, the same as, or lower than the source frequency. Explain your reasoning using the properties of the Doppler effect.
Solution and grading rubric:
- p = 10/10:
Correct. Either of two answer suffices: (a) Doppler shifts only occur if the source and/or frequency move relative to each other, thus it can be shown from the frequency shift equation that the numerator and denominator are both equal to one, and thus no shift occurs; or (b) the wind increases the wavelength of the waves traveling from the source to the observer, but these spaced-out wavefronts reach the observer quicker, resulting in no net change in frequency. Explanation includes the frequency shift equation (with v_obs = 0, v_source = 0, but with v_sound increased) and/or illustration of wavefronts. - r = 8/10:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. - t = 6/10:
Nearly correct, but argument has conceptual errors, or is incomplete. Typically motivates the observed frequency as being higher or lower than the source frequency. - v = 4/10:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. - x = 2/10:
Implementation/application of ideas, but credit given for effort rather than merit. - y = 1/10:
Irrelevant discussion/effectively blank. - z = 0/10:
Blank.
Grading distribution:
p: 11 students
r: 3 students
t: 21 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students
A somewhat fanciful, overwrought sample of a "p" response (from student 1337) is shown below:
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