# Transformation of a Vector

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• Feb 2nd 2013, 09:52 AM
Vesnog
Transformation of a Vector
Greetings,

My question is from the book "Tensor Analysis" by Barry Spain. I am asked to show that what the components of a vector become upon transforming from rectangular Cartesian coordinates to polar coordinates. I have attached the question in jpeg format. I have came up with a solution but the angular component in my solution is r^2 times the angular component given in the book. I have checked some other books on this subject and found out that both the solution given in the attachment and the one I found exist. I am pretty confused about this, and I assume that this book is wrong or I am doing a terrible mistake. I will be grateful if someone can provide some insight.

Note: I found it appropriate to post this in differential geometry section since it is related with tensors somehow.
• Feb 3rd 2013, 06:59 AM
Vesnog
Re: Transformation of a Vector
Any ideas? I can post my solution method if it is necessary.
• Feb 3rd 2013, 10:35 AM
ILikeSerena
Re: Transformation of a Vector
Hi Vesnog! :)

See here for a derivation.
• Feb 3rd 2013, 02:31 PM
Vesnog
Re: Transformation of a Vector
Okay my derivation was similar and it is included in many textbooks but my problem is that there is an additional factor of r^2 in the angular part of the vector in my solution and in Wikipedia's solution. My question is this, if it is not clear I can explain it again.
• Feb 3rd 2013, 02:43 PM
Vesnog
Re: Transformation of a Vector
The solution for this transformation is also given as in my book on a book available through Google. I am attaching a screenshot to pinpoint my point.
• Feb 3rd 2013, 03:02 PM
ILikeSerena
Re: Transformation of a Vector
Ah, now I understand what you mean.
But it is not a factor of $r^2$, but a factor of $r$.

When we use polar coordinates $(r,\theta)$, a small change is expressed as $\mathbf{dr} = dr \mathbf{\hat r} + r d\theta \boldsymbol{\hat \theta}$.
This change is identified by $(dr, d\theta)$.
Note that the vector formula has an extra r in it where the change in angle is identified, but when we identify them as a coordinate pair, the r is left out.
• Feb 3rd 2013, 03:34 PM
Vesnog
Re: Transformation of a Vector
Thanks for your replies. First of all that should be a factor of r sorry for that, I am glad you got my point; however, I still could not understand your last statement(sentence).
• Feb 3rd 2013, 11:28 PM
ILikeSerena
Re: Transformation of a Vector
It's an arbitrary decision to take a factor r out.
It does not have any particular meaning or significance.
• Feb 4th 2013, 02:09 AM
Vesnog
Re: Transformation of a Vector
It may be significant because of dimensional analysis by the way the book should explicitly give a reason why it ignores r in my opinion. There may be a more subtle reason behind this.