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Related Rates: Man and Spotlight

  1. Nov 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A 2m tall person walks toward toward a wall at 1.6 m/s. A spotlight 12 m away from the wall shines at it. How fast is the length of his shadow on the wall decreasing when he is 4m away from the wall?

    I tried a few different things with trig, none of which worked.
     
  2. jcsd
  3. Nov 5, 2009 #2

    lanedance

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    like what?
     
  4. Nov 5, 2009 #3
    Well, actually I couldn't figure out how to setup the equation to begin with.
     
  5. Nov 5, 2009 #4

    lanedance

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    ok so start by drawing a picture, and try and relate the distnace of the man from the light or wall to the height of the shadow - think similar triangles
     
  6. Nov 5, 2009 #5
    Yeah, that's where I run into problems.
     
  7. Nov 5, 2009 #6

    lanedance

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    ok so what do you need a hand with?
     
  8. Nov 5, 2009 #7
    How can I go about setting up the problem? How can I go about finding an equation to solve? I understand how to solve these problems, but not how to go about setting up their equations.
     
  9. Nov 5, 2009 #8
    Think about the spot light creating a triangle and then insert the man to create a second smaller triangle. Using the numbers you have in the question, you can get your equation from that by setting the two similar triangles equal to each other - I can't remember the exact name for that, sorry. Ex. base/height of triangle one = base/height of triangle two then cross multiply and solve for your unknown that may or may not start with an s.
     
  10. Nov 7, 2009 #9
    I managed to get it. Sorry it took so long to get back. Thanks for your help!
     
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