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Thread: Minimal Distance Between Two Spheres

  1. #1
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    Minimal Distance Between Two Spheres

    The minimal distance between any point on the sphere1 $\displaystyle (x-2)^{2}+(y-1)^{2}+(z-3)^{2}=1$ and $\displaystyle (x+3)^{2}+(y-2)^{2}+(z-4)^{2}=4$ (sphere2) is...?

    Looking at the range of values
    $\displaystyle 1 \leq x \leq 3,
    0 \leq y \leq 2,
    2 \leq z \leq 4$ for sphere1 and
    $\displaystyle -5 \leq x \leq -1,
    0 \leq y \leq 4,
    2 \leq z \leq 6$ for the sphere2, I decided that the minimal distance would occur at x=1 for sphere1 and x=-1 for sphere2 because these are the x-coordinates that are closest while still being on their respective spheres.

    If I plug those in, it gives me
    sphere1: $\displaystyle (y-1)^{2}+(z-3)^{2}=0$ and
    sphere2: $\displaystyle (y-2)^{2}+(z-4)^{2}=0$. I think I should be doing something with these equations but I'm not sure what that would be.

    I also decided that the minimal distance must lie along the line connecting the centers of the circles, so I used (2,1,3) - (-3,2,4) = <5,-1,-1> as a position vector and the coordinates of the center of sphere1 to give me an equation of a line: (2,1,3) + t<5,-1,-1>.

    This is where I'm stuck. Using x=1 and then x=-1, I solved x=2+5t for t, then plugged these values back into the equations y =1-t and z=3-t to get the y and z coordinates, but the distance between the two points is incorrect (I have the correct answer; it's $\displaystyle 3(\sqrt{3}-1)$. I calculated $\displaystyle (1, \frac{6}{5},\frac{16}{5})$ and $\displaystyle (-1, \frac{8}{5},\frac{18}{5})$ for the points, and the distance between them is $\displaystyle \frac{6}{5}\sqrt{3}$.
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  2. #2
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    Quote Originally Posted by StarKid View Post
    The minimal distance between any point on the sphere1 $\displaystyle (x-2)^{2}+(y-1)^{2}+(z-3)^{2}=1$ and $\displaystyle (x+3)^{2}+(y-2)^{2}+(z-4)^{2}=4$ (sphere2) is...?
    The distance between the centers is $\displaystyle \sqrt{27}$.
    The sum of the radii is $\displaystyle r_1+r_2=1+2=3$.
    So what is the minimum distance between the spheres?
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  3. #3
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    Quote Originally Posted by Plato View Post
    The distance between the centers is $\displaystyle \sqrt{27}$.
    The sum of the radii is $\displaystyle r_1+r_2=1+2=3$.
    So what is the minimum distance between the spheres?
    I see...the minimum distance is the distance between the centers minus the sum of the radii.
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