Physics 5A Midterm 2, Spring Semester 2008
Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 11.3
[3.0 points.] The intensity of a sound wave from a jet airplane as it is taking off is 12 Watts/m^2 at a distance of 10 m. What happens to the intensity of the sound waves as they travel out to a distance of 20 m from the jet airplane?
(A) It decreases by a factor of 1.4.
(B) It decreases by a factor of 2.0.
(C) It decreases by a factor of 4.0.
(D) (The intensity of the sound waves remains the same.)
Correct answer: (C)
The intensity of a spherical sound wave, with no reflections or absorption, is inversely proportional to the square of the radius, as measured from the (isotropic) source. Thus doubling the distance from the source results in reducing the intensity by a factor of four.
Student responses
Sections 4987, 4988
(A) : 7 students
(B) : 10 students
(C) : 15 students
(D) : 3 students
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