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Homework Help: The Inverse Square Law

  1. May 21, 2014 #1
    1. The problem statement, all variables and given/known data
    Problem One: Two kilometres away from a point source of infrared waves, the intensity is 4 Mw−2. Calculate the intensity 1m away from the source.

    Problem two: Light from a candle has an intensity of 20.0 units when a meter is placed 3.0m away. What is the reading on the light meter when it is brought to 1.0 m away?

    2. Relevant equations

    3. The attempt at a solution
    Yeah, about that. I'm not really sure where to start. I mean, I have the answers with me, and I think I am able to fill in the values for the inverse square law, but I don't know how to manipulate the equation with fractions to get the answer that I want. I always struggle here, because I ****ing hate fractions and I can't seem to get one thing write with them. I'd like to see the solution with respect to the given formula above please.
  2. jcsd
  3. May 21, 2014 #2


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    Look at the first problem. You are given the intensity and distance for one measurement; let those be ##I_1## and ##d_1## respectively. You are also given another distance, and you want to find the intensity at that spot; let those be ##d_2## and ##I_2##. You have to solve the equation you have above for ##I_2##, and then plug in the numbers you have to get your answer.
  4. May 22, 2014 #3
    If your problem is with fractions than I think you should go back to basics and solve many more fraction problems from some introductory text that includes lots of examples and exercises.
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