Inverse Square Law: Calculating Intensity at Different Distances

[emtis]

Homework Statement

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?

Homework Equations

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.

 

[tms]

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.

 

[dauto]

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.