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Euclidean Geometry Question

  1. Oct 29, 2005 #1
    Suppose you have a smooth parametrically defined volume V givin by the following equation.

    f(x,y,z,w)= r(u,s,v) = x(u,v,s)i + y(u,v,s)j +z(u,v,s)k + w(u,v,s)l

    Consider the vectors ru=dr/du, where dr/du is the partial derivitive of r with respect to the parameter u. Similarly, rv = dr/dv, rs=dr/ds are the partial derivitives of r with respect to the parameters v and s, respectively.

    I presume that ru(u0,v0,s0), rv(u0, v0, s0), and rs(u0,v0,s0) form a three dimensional parallelpiped that represents the rate of change of the three dimensional surface volume in four dimensional Euclidean space.

    The angle theta between ru and rv is arccosine((ru*rv)/(|ru||rv|))

    Similarly, the angle phi between ru and rs is arccosine((ru*rs)/(|ru||rs|).

    The height h1 of the parallelogram p1 formed by ru and rv is the magnitude of the projection of rv onto the perpindicular of p1 and is equal to
    |rv|sin(theta). The area A1 of this parallelogram is
    |ru|*h1 = |ru||rv|sin(theta).

    The height h2 of the parallelogram p2 formed by ru and rs is the magnitude of the projection of rs onto the perpindicular of p2, and is equal to |rs|sin(phi)
    The area of A2 of this parallelogram is |ru|*h2 = |ru||rs|sin(phi).

    The volume of the parallelpiped V = the area of either of the parallelograms times the height of the other parallelgram.

    A2*(h1) = A1*(h2) =|ru||rv||rs||sin(phi)*sin(theta)|

    Based on this, I conjecture the following:

    If a smooth parametrically defined volume V is givin by the following equation:

    r(u,s,v) = x(u,v,s)i + y(u,v,s)j +z(u,v,s)k + w(u,v,s)l

    Where (u,s,v) are elements of E, and

    V is covered just once as (u,v,s) varies throughout the parameter domain E, then the "Surface Volume" is

    the tripple integral over E of =|ru||rv||rs||sin(phi)*sin(theta)|dV

    Where ||sin(phi)*sin(theta)|is a constant.

    Does this sound accurate?


  2. jcsd
  3. Oct 30, 2005 #2
    Where ||sin(phi)*sin(theta)|is a constant.

    Oops! ||sin(phi)*sin(theta)|is not a constant!

    it's |arccosine((ru*rv)/(|ru||rv|))*arccosine((ru*rs)/(|ru||rs|)|.
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