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I am completely drawing a blank! Exam tomorrow, help?

  1. Jan 19, 2005 #1
    Hey, in the process of searching for an explanation for this problem tonight, I stumbled across this forum. I usually do well in physics, but our semester exam is tomorrow and, as I'm reviewing, I am drawing a complete blank on everything! I'll just give one problem, to see if someone could get me on the right track again.

    Thanks a lot -- I am sure this is ridiculously easy, I just can't seem to pull it out of my brain.

    A firework explodes in the sky releasing 20 W of sound energy in a time of .05 s. John is standing 80 m away at the time when he hears the bang. The intensity of sound at John's ear will be about:

    a) 1.5 x 10^-3 Wm^-2
    b) 2.5 x 10^-3 Wm^-2
    c) % x 10^-3 Wm^-2
    d) 2.0 x 10^-2 Wm^-2

    Thanks a bunch, again.
    ~ Apodidomai
  2. jcsd
  3. Jan 20, 2005 #2
    The sound source can be assumed to be spherically isotropic and to send out a "spherical" wave. The amplitude of such a wave falls of as (1/r) (r = radial distance from source). Additonally, the intensity of the wave at a certain r is defined as the ratio of the power of the source to the area of an imaginary spherical surface centered at the source with a radius r. Does that help? (Refer to Physics, by Resnick and Halliday; chapter on waves => spherical waves).
  4. Jan 20, 2005 #3
    lol. i totally know how that feels.... my semester exam was today and last night, i just coudnt answer any of the physics questions even though they were really easy and i would have been able to answer them on a good day :P
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