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Math Help - Tough problem... Vectors, points, and a cube

  1. #1
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    Tough problem... Vectors, points, and a cube

    Problem:
    A strange building has been built to look like part of a cube sticking out of the ground. You are required to find the 3d coordinates for points B and D, given the points A(13,30,9) and C(2,4,17). Points B and D are at equal height above the ground. The ground coincides with the x,y-plane



    This picture provides two views of the building.

    Relevant Equations:
    Vector properties (dot product, cross product, length of vector v = |v|.

    Anyone know how to approach this?
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  2. #2
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    I haven't worked this through fully... but here are my thoughts:

    - If B and D are at equal height above the ground, they will have the same z-coordinate (why?). So, z = z_3

    - The three vectors that define each corner of the cube are mutually perpendicular. So the vectors  \vec{AD} and \vec{AB} are perpendicular, as are \vec{CD} and \vec{BC}. Can you think about how you might use the vector dot or cross product to process this information?

    Hope that helps a little.
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  3. #3
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    If the points B and D have the same z-coordinate then so does any point on the line BD. In particular, the midpoint of BD, which is also the midpoint of AC, has that same z-coordinate. But the coordinates of the midpoint of AC are given by \tfrac12(13,30,9) + \tfrac12(2,4,17). All you have to do is to compute that third coordinate.
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  4. #4
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    Elementary Solution

    The distance AC=29.343. Since this is the hypotenuse of a 45-45-90 triangle, the sides of the cube are 20.748.

    Since AB and AD are both this length, B and D must satisfy the equation (x-13)^2+(y-30)^2+(z-9)^2=20.748^2. Similarly, Since CB and CD are both this length, B and D must satisfy the equation (x-2)^2+(y-4)^2+(z-17)^2=20.748^2

    Setting these two equations equal to each other, we get 22x+52y-16z=841. Since B and D are the same height (same value for z), they must be the same height as AC's midpoint (try to visualize this). The z-value of AC's, and therefore B's and D's coordinates are therefore \frac12(9+17)=13. Letting z=13 in all further equations, we arrive at 22x+52y=1049, or y=20.173-.423x.

    Now we pick the simpler of the two equations above and substitute y and z in terms of x: (x-2)^2+(16.173-.423x)^2+(13-17)^2=20.748^2, or 1.178x^2-16.503x-148.934=0. The discriminant of this quadratic is 31.22, therefore the two possible values of x are 20.24 and -6.24. Plugging these back in for y, we get y=11.61 or 22.81.

    So our final answers are B=(20.24,11.61,13) and D=(-6.24,22.81,13)

    Aside from a few possible algebraic and/or arithmetic and/or rounding errors, this is (in my opinion) the most elementary approach, if not the fastest. Make sure to run through these equations to make sure you understand them and see where their solutions come from.
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  5. #5
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    Quote Originally Posted by Media_Man View Post
    The distance AC=29.343. Since this is the hypotenuse of a 45-45-90 triangle, the sides of the cube are 20.748.
    Could you tell me how you got the length of AC? By finding the AC vector and then |AC| to find it's length, I'm getting 29.698. Trivial difference, I know, but I have to be spot on. Are you rounding before your final result?
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  6. #6
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    Arithmetic?

    Quote Originally Posted by Ares_D1 View Post
    Could you tell me how you got the length of AC? By finding the AC vector and then |AC| to find it's length, I'm getting 29.698. Trivial difference, I know, but I have to be spot on. Are you rounding before your final result?
    \sqrt{(13-2)^2+(30-4)^2+(9-17)^2}= \sqrt{(11)^2+(26)^2+(-8)^2}=\sqrt{121+676+64}=\sqrt{861}=20.3428015

    You are getting 29.69848481=\sqrt{882}. Check your math.
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  7. #7
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    Quote Originally Posted by Media_Man View Post
    \sqrt{(13-2)^2+(30-4)^2+(9-17)^2}= \sqrt{(11)^2+(26)^2+(-8)^2}=\sqrt{121+676+64}=\sqrt{861}=20.3428015

    You are getting 29.69848481=\sqrt{882}. Check your math.
    Yep, exactly what I did. You've just rounded before your final result, hence the discrepancy.

    Not sure if I understand the second sentence in your first post, but it's probably because it's 3:30am here. :O I'll have a look over it tomorrow and ask questions if need be.

    Thanks so much for the help everyone.
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  8. #8
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    Going Further

    hellohello123 -

    A = (13.0492, 30.9498, 9.01115),
    B = (21.1958196, 12.05024361, 13.187175),
    C = (1.97687, 4.68868, 17.3632),
    D = (-6.169749596, 23.58823639, 13.187175),
    O = (7.513035, 17.81924, 13.187175)

    At the moment I'm having trouble finding the 4 points A', B', C' and D' where the cube's corners intersect the ground. Another two properties I need to find are the area that this A'B'C'D' plane on the ground encloses and also what volume of the cube is above or below the ground. I'm so sorry to be bothering you but any advice at all in approaching this would be of huge benefit to me.
    Finding Coordinates of A',B',C',D'

    Find the normal vector N of the plane ABCD using cross product: N=ABxAD=(-46,-108,-421)

    Each line AA',BB',CC',DD' has the same direction as N, so the parametric equations are A'(t)=A+Nt, B'(t)=B+Nt, C'(t)=C+Nt, D'(t)=D+Nt. These four lines intersect the plane A'B'C'D' when their z-value is zero (by construction). Therefore set A_z'(t)=A_z+N_zt=0 to solve for t, and plugging t back into A'(t) gives you the other two coordinates of A'. Repeat to find B',C',D'.

    Finding the Area of A'B'C'D'

    The area of a parallelogram Area=|A'B'xA'D'|, the magnitude of the cross product of two adjacent sides.

    Finding the Volume of the contained area

    The volume of the figure is simply the area of ABCD times the "height" OO'. So, using the parametric equation technique described above, O'(t)=O+Nt, find the coordinates of O', use the length of OO' as your height, and multiply by S^2, where S is the length of the side of the top face of the cube.
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  9. #9
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    Thank you so very much, I will get working on this ASAP.
    Last edited by hellohello123; August 14th 2009 at 05:47 AM.
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  10. #10
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    Quote Originally Posted by Media_Man View Post
    hellohello123 -
    Finding the Volume of the contained area

    The volume of the figure is simply the area of ABCD times the "height" OO'. So, using the parametric equation technique described above, O'(t)=O+Nt, find the coordinates of O', use the length of OO' as your height, and multiply by S^2, where S is the length of the side of the top face of the cube.
    Media_Man,

    I managed to get the rest of the problem, but not sure I'm following this bit. Specifically, where the points O and O' come from. How do I get their values so I can solve the parametric for t?
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  11. #11
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    Sorry, I thought we used that in a previous thread. I'm using the point O to represent the center of the cube face, easily gotten by taking the midpoint of AC, so O=(7.5,17,13). Now O' is the center of the "shadow" shape A'B'C'D'. Therefore, find O' the same way you found the other "shadow" points, using O'(t)=O+Nt, setting the z coordinate equal to zero to find t, and plugging t back into O'(t) to find all three points. If you imagine turning this object upside down, you have a perfect right cube sitting on its face with its top sliced off. The height at which this slice occurs (average height) determines the remaining volume. Thus the height can be gotten as the distance between the centers of the top and bottom.
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  12. #12
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    Quote Originally Posted by Media_Man View Post
    hellohello123 -



    Finding Coordinates of A',B',C',D'

    Find the normal vector N of the plane ABCD using cross product: N=ABxAD=(-46,-108,-421)

    Each line AA',BB',CC',DD' has the same direction as N, so the parametric equations are A'(t)=A+Nt, B'(t)=B+Nt, C'(t)=C+Nt, D'(t)=D+Nt. These four lines intersect the plane A'B'C'D' when their z-value is zero (by construction). Therefore set A_z'(t)=A_z+N_zt=0 to solve for t, and plugging t back into A'(t) gives you the other two coordinates of A'. Repeat to find B',C',D'.

    Finding the Area of A'B'C'D'

    The area of a parallelogram Area=|A'B'xA'D'|, the magnitude of the cross product of two adjacent sides.

    Finding the Volume of the contained area

    The volume of the figure is simply the area of ABCD times the "height" OO'. So, using the parametric equation technique described above, O'(t)=O+Nt, find the coordinates of O', use the length of OO' as your height, and multiply by S^2, where S is the length of the side of the top face of the cube.
    would you be able to explain to me how you got (-46,-108,-421) for the normal vector? i know how to use the cross product. but i think my AB and AD values might be different.
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