1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Parameters for polar conics

  1. Nov 14, 2013 #1
    1. The problem statement, all variables and given/known data

    Hi wondering what the directrix is for this

    2. Relevant equations

    r = (20)/ (2+sin(theta))

    3. The attempt at a solution

    I factored denominator so it read 2(1+ (1/2)sin(theta)) ...So I said e times d = (2/20) and I got d = (1/5) because e is (1/2). Doesn't make sense though seems to small because that lies inside of the thing when I graph it.
  2. jcsd
  3. Nov 14, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    This bit you did OK - try working on the whole fraction though.
    $$r=\frac{20}{2+\sin\theta} = \frac{10}{1+\frac{1}{2}\sin\theta}=\frac{ed}{1+e\sin\theta}$$

    So ##e=\frac{1}{2}## right? And ##ed=\cdots##?
  4. Nov 15, 2013 #3
    d = 20? ed = 10 = (1/2) = 10
  5. Nov 15, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Um... that is just nonsense. eg. 10 ≠ (1/2)

    You have to use reasonable grammar when you write sentences in any language.
    Math is also a language - therefore you should take some trouble to make sure the math/number sentences you write down actually mean something.

    Want to try again?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted