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Homework Help: Coordinates transformation

  1. Aug 26, 2012 #1
    1. The problem statement, all variables and given/known data

    The x-y coordinates are being transformed into the u-v coordinates.

    Based on the diagram, u lies along x while v makes an angle α with x.

    3. The attempt at a solution

    The answer defined u and v weirdly..


    x = u


    y = v sin α


    Attached Files:

  2. jcsd
  3. Aug 26, 2012 #2


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    No. Consider the position vector [itex]\mathbf{r}= x\mathbf{e}_x + y\mathbf{e}_y = u\mathbf{e}_u + v\mathbf{e}_v[/itex]:

    You have [itex]\mathbf{e}_u = \mathbf{e}_x[/itex] since the two axes are parallel, but [itex]\mathbf{e}_v[/itex] has both a vertical and a horizontal component and is given by [itex]\mathbf{e}_v = \cos\alpha \mathbf{e}_x + \sin\alpha \mathbf{e}_y[/itex]. Plugging this into the position vector definition gives [itex]x\mathbf{e}_x + y\mathbf{e}_y= u \mathbf{e}_x + v( \cos\alpha \mathbf{e}_x + \sin\alpha \mathbf{e}_y)[/itex], which gives you the relations in your image.
  4. Aug 26, 2012 #3
    Also, does the region of integration R change if we change the variables from (x,y) to (u,v)?

    According to the answer, the region R → R', where R' is only σ/2∏ of the original R..

    Attached Files:

  5. Aug 26, 2012 #4


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    No, that's not what they are saying. Read it again more carefully, what they are actually claiming is that [itex]\int_{0}^{\infty} \int_{0}^{\infty} e^{-r^2} \left| \frac{\partial(x,y)}{\partial(u,v)} \right|dudv = \frac{\alpha}{2\pi} \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} e^{-r^2}dxdy[/itex]

    The integral on the left is only over positive [itex]u[/itex] & [itex]y[/itex], while the integral on the right is over all (R2) space.
  6. Aug 26, 2012 #5
    Yup, if you only consider x,y,u,v > 0,

    it would be α/(∏/2) for ∫ 0 to infinity..
  7. Aug 26, 2012 #6
    I dont really understand what you mean...

    My main problem here is why do they define u and v as such in the picture?

    I thought u and v are defined when you drop a perpendicular line onto the axis?

    And it's pretty clear that the lengths u, v they define are shorter than the ones in my picture..
  8. Aug 26, 2012 #7
    That's the case when your coordinate lines are perpendicular. When they are not, you get what you see in this picture.
  9. Aug 26, 2012 #8
    Are they defined this way?
  10. Aug 26, 2012 #9
    A coordinate system (on an plane) is defined by its origin and unit vectors [itex]\vec{a}[/itex] and [itex]\vec{b}[/itex]. Any point [itex]\vec{p} = u\vec{a} + v\vec{b}[/itex]. [itex]u[/itex] and [itex]v[/itex] are coordinates. Now if the coordinate unit vectors are not perpendicular, what do you get? Try it on a piece of paper.
  11. Aug 26, 2012 #10
    Ah, using vectors everything seems much simpler now! Thank you! :smile:
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