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Homework Help: Finding the Vertex Coordinates of a Rectangle In Cartesian Space

  1. Aug 20, 2009 #1
    I am hoping to find the coordiantes of all 4 vertices when the rectangle is in any orientaion knowing the length l, the width b, the coordinate of its center mark (xcen,ycen), and the coordinate of vertex A as shown below:

    This is NOT HOMEWORK so although I think it is possible to do, I am not sure that it is.

    Any ideas?

    I am thinking of using the vector that points from A to the center somehow... I know that if I double its length then I have arrived at the vertex C.... but how to extract those coordinates, I cannot see.

    EDIT: Here is a drawing that better illustrates what I am thinking. The Blue Rectangle is the one I want to to find the vertices for. I know all information in blue.

    The Black Rectangle shares the same A vertex and is in what I have DEFINED to be standard reference position (SRP).

    I could find the angle of the vector that points from A to the center rc of the black rectangle and compare it with the angle of that of the blue rectangle r'c

    I know that the difference [itex]\theta - \theta '[/itex] should be the angle that all vertices should carve out. I just can't see how to make the actual calculations of their cartesian coordinates?

    Last edited: Aug 20, 2009
  2. jcsd
  3. Aug 20, 2009 #2
    Looking around online and a Rotation Matrix seems promising.

    Though I am finding a lot of definitions of a Rotation Matrix, I am not finding many practical examples... so I am not exactly sure what it does.
  4. Aug 20, 2009 #3
    Okay :smile: Here is where I am at: Updated Diagram for reference:


    rc and r'c are the vectors from A to the centers.

    In order for all vertices to get from the standard ref posotion to the new positions, they must all rotate through the angle phi correct?

    If A=A' is locate at the point (x0, y0), then we have the points

    A(x0, y0)
    B(x0, (y0+b)) *Taking down as +Y and Right as +X
    C((x0+L), (y0+b))
    D((x0+L), y0)

    Now to use a Rotation Matrix to get the new coordinates of A' B' C' D'
    I am a little confused.

    Do I use the coordinates? Or the vector components of rBA,
    rAD, etc... ?

    FOR EXAMPLE: If I am looking at the vectors rBA and r'BA

    And I know that to get from rBA to r'BA I rotated through the angle Phi. How do I get the new coordiantes of B out of the deal? :confused:
  5. Aug 20, 2009 #4


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    While this can be done using rotations, I don't think it's necessary.

    We know that B is a distance b from A, and a distance rc from the center. In other words, it's at the intersection of two circles. You'd just need to set up the equations and solve them.

    One problem I see, which is that there are two solutions for the location of B. You'll need another condition or some way to specify which solution is the desired one.
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