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Homework Help: Geodesics and straight lines on a surface

  1. Jun 1, 2010 #1
    1. The problem statement, all variables and given/known data

    Let [tex]\gamma[/tex] be a stright line in a surface M. Prove [tex]\gamma[/tex] is a geodeisc

    3. The attempt at a solution

    In a plane we know a straight line is the shortest distance between two point. I am not sure if this applies to straight lines on a surface.

    Further more, there is a theorem that says that if [tex]\gamma[/tex] is a unit speed curve and the shortest distance between two points P= [tex]\gamma (a)[/tex] and [tex]Q=\gamma (b)[/tex]then it is a geodesic.

    But i do not know how to show some arbitrary straight line is unit speed or if this approach is even valid.

    Any help appreciated.
  2. jcsd
  3. Jun 1, 2010 #2


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    Science Advisor
    Homework Helper

    You know that a straight line is the shortest distance between two points in the Euclidean space that contains the surface. An arbitrary straight line doesn't have to be unit parameterized, but you can certainly parameterize it to be unit length.
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