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A difficult (for me) 3d shapes problem

  1. Mar 24, 2012 #1
    If I have a cylinder with a radius r and an axis that passes through point b with the
    direction of vector n, show that its equation can be written in any of the following forms:
    1) ||(p-b) X n|| = r
    2) (p - b) X n = r.e (where e s ia unit vector orthogonal to n)
    3) ||(p-b) - ((p-b).n)n|| = r
    . = vector-vector dot product
    X = vector-vector cross product

    Thanks in advance for any guide given...
     
  2. jcsd
  3. Mar 25, 2012 #2

    tiny-tim

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    hi ydan87! :wink:

    start by translating each of 1) 2) and 3) into ordinary english …

    what do you get? :smile:
     
  4. Mar 25, 2012 #3
    Tiny tim :eek:
    1) the length of the vector you get by the cross product of the vector from point p to point b and normal n equals to r, the radius of the cylinder.
    2) the vector you get by the cross product above equals to the vector you get by multiplying scalar r with vector e, which is a unit vector orthogonal to n.
    If you guide me through those i'll be ok with 3.

    Is it clearer now?
     
  5. Mar 25, 2012 #4

    tiny-tim

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    ok :smile:

    now also use the words "cos" or "sin" :wink:
     
  6. Mar 25, 2012 #5
    Can you please explain what you mean? I can't give you the parametric representation if that's what you mean....
     
  7. Mar 26, 2012 #6

    tiny-tim

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    i meant explain what the cross product is, instead of just saying "cross product"! :smile:
     
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