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Homework Help: Spherical vector problem

  1. Sep 24, 2005 #1
    My friends and I have been trying to work this one out all night, but to no avail.
    If a curve has the property that the position vector r(t) is always perpendicular to the tangent vector r'(t), show that the curve lies on a sphere with center the origin.

    We know the dot product of r(t) and r'(t) = 0 or that r(t) cross r'(t) equals the multiplication of their magnitudes but to go about showing that it is a sphere because of this is causing a great deal of difficulty. Any help would be appreciated
     
  2. jcsd
  3. Sep 25, 2005 #2

    Tide

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    If [itex]\vec r \cdot \frac {d \vec r}{dt} = 0[/itex] then [itex]\frac {d}{dt} r^2 = 0[/itex].
     
  4. Sep 25, 2005 #3

    HallsofIvy

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    Or, to put what Tide said in different words, if the derivative of a vector is always perpendicular to the vector, the vector has constant length.
     
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