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Math Help - Light Intensity rate

  1. #1
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    Exclamation Math Questttttttt

    An object is moving away from the light source at a rate of 4 units/s. Find the rate of change of the intensity of illumination when it is 2 units from the light source.

    yeah, this is my answer:
    rate of change = k(1/unit˛xsecond)
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by mathletes
    An object is moving away from the light source at a rate of 4 units/s. Find the rate of change of the intensity of illumination when it is 2 units from the light source.

    yeah, this is my answer:
    rate of change = k(1/unit˛xsecond)
    The intesity of illumination as a function of distance r from a point source is:

    I(r)=I(1)/r^2,

    where I(1) is the intensity of illumination at a distance of 1 unit from the source.

    Now:

    \frac{d}{dt} I(r)=I(1)\frac{-2}{r^3}\ \frac{dr}{dt}.

    So now you only need to plug in r=2 and \frac{dr}{dt}=4 to get the answer, in units of
    (whatever you are using as units of intensity)/s.

    RonL
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  3. #3
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    Smile

    thank you, i obviously didn't know what i was doing.
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