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Homework Help: Concave Mirrors

  1. Sep 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A concave mirror of focal length 20cm forms an image which is three times the height of the object. Find the distance of the object from the mirror if: 1: The image is real
    2: The image is virtual

    2. Relevant equations
    1/u + 1/v = 1/f
    m = v/u

    3. The attempt at a solution
    f = 20cm
    u = ?
    v = ?
     
  2. jcsd
  3. Sep 9, 2010 #2

    Redbelly98

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    Welcome to Physics Forums.

    The magnification tells you the ratio v/u, though it could be either positive or negative.
     
  4. Sep 10, 2010 #3
    Sorry, but I still don't understand :confused:
     
  5. Sep 10, 2010 #4

    Redbelly98

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    Okay, think about it this way: based on the given information, we can say that
    v = ____ × u
    where you must fill in the blank with a number.
     
  6. Sep 10, 2010 #5
    Is the object <answer deleted by moderator> away?
     
    Last edited by a moderator: Sep 10, 2010
  7. Sep 11, 2010 #6
    I have the answers on the back of the book but cant figure out how to get it
     
  8. Sep 11, 2010 #7

    Redbelly98

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    And I will try one more hint. You wrote this equation earlier:
    What is the value of the magnification m, based on the information given in the problem statement?
     
  9. Sep 11, 2010 #8
    Magnification is 3 ??
     
  10. Sep 11, 2010 #9
    If I divide 3 into 20 I get 6.666667. If I add 6.6667 to 20, is that how I get the real height, then take 6.66667 from 20 to get virtual height?
     
  11. Sep 11, 2010 #10

    Redbelly98

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    Uh, I don't think so. You get the answer by working with the two equations you wrote earlier.
    Yes, good. So your two equations your wrote in Post #1 become:

    1/u + 1/v = 1/f
    ___ = v/u​

    (Fill in the blank to get the complete equation.)
     
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