What is this vector problem asking?

  • Thread starter reddawg
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  • #1
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Homework Statement


If r = <x, y, z> and r0 = <x0, y0, z0>, describe the set of all points (x, y, z) such that the magnitude of r – r0 = 4.


Homework Equations





The Attempt at a Solution


I don't know what this problem is asking to even attempt a possible solution. Please help?
 

Answers and Replies

  • #2
PeroK
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If you start by trying r0 = <0, 0, 0> does that help? Can you see what that means for r?
 
  • #3
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Trying that I get x2 + y2 + z2 = 16 after finding the magnitude, but I still can't see what that does to r.
 
  • #4
D H
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What kind of object does that equation describe?
 
  • #5
PeroK
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What if you lose the z and reduce it to two dimensions: $$x^2 + y^2 = 16$$?
 
  • #6
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Including the z^2, the equation represents a sphere with radius 4. Without the z^2 it's a circle.
 
  • #7
PeroK
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Good. And, if r_0 is the point <x0, y0, z0>?
 
  • #8
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I'm guessing that the reverse would be true making the components of r_0 negative, however squaring those would make that irrelevant. This is where I'm not following.
 
  • #9
PeroK
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Think geometrically.
 
  • #10
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So r_0 is the center/origin of the sphere/circle?
 
  • #11
PeroK
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Yes, that's it. $$|r - r_0| = 4$$ is the vector equation for a sphere of radius 4, centred at r0.

It's equivalent to $$(x-x_0)^2 + (y-y_0)^2 + (z-z_0)^2 = 16$$
 
  • #12
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Wow I way over thought this.

Thanks PeroK.
 

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