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Polar conics

  1. Nov 14, 2013 #1
    1. The problem statement, all variables and given/known data
    I want to know if I did these right.

    Write a polar equation of a conic with the focus at the origin and the given data.
    2. Relevant equations

    r = (ed) /(1+- cos(theta)) and r = r = (ed) /(1+- sin(theta))


    3. The attempt at a solution

    Parabola , directrix x = -3 I came up with r = -3/ (1 -cos(θ))

    Hyperbola, eccentricity 3, directrix x = 3
    I got 9/(1+3cos(θ))

    What do you think OK?
     
  2. jcsd
  3. Nov 15, 2013 #2

    Simon Bridge

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    Did you try plotting them?
    For instance, a parabola with directorix -3 ... what would a sketch of it look like?
    Does this match up with some values - like what would r be at zero angle?
    Does that match up with your equation?
     
  4. Nov 15, 2013 #3
    Yeah I plotted them. Idk tho
     
  5. Nov 15, 2013 #4

    Simon Bridge

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    And? .... what did you find out?

    Idk <looks it up> "I don't know"?
    What don't you know?

    Tho <looks it up>
    1. From Middle English tho, tha, from Old English þā (“the, those”, plural), from Proto-Germanic *þai (“those”), from Proto-Indo-European *to-, *só (“that”).
    2. abbv. Thor Industries
    3. Internet slang - short for "though"
    <sigh>

    If you were to sketch a parabola with a directorix at x=3,
    - is the parabola oriented with the +y axis? The +x axis? something else?
    - where is the focus?
    - what sort of value should r have at easy angles like 0, 90, 180, 360?
    - will r ever be negative?

    How does this compare with the results fro your equation: ##r=-3/(1-\cos\theta )## ?

    If you don't know the basic properties of a parabola, without using the equation, then you need to learn them.
    http://en.wikipedia.org/wiki/Conic_section
     
    Last edited: Nov 15, 2013
  6. Nov 17, 2013 #5
    Hyperbola, eccentricity 3, directrix x = 3
    I got 9/(1+3cos(θ)) This one is to the right. The some. The question say's " Write a polar eq. of a conic with the focus at the origin and the given data".
    So, this one OK?
     
  7. Nov 18, 2013 #6

    Simon Bridge

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    Well ... write down a similar list to before and check it off.
    What do you get?
     
  8. Nov 18, 2013 #7

    Simon Bridge

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    Well ... write down a similar list to before and check it off.
    What do you get?
     
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