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What is this? Absolute value derivative? Magnitude?

  1. Sep 17, 2009 #1
    I'm looking at this problem below:

    http://apthtml.com/images/deriv.png [Broken]

    The first part I understand - a derivative of some vectors, not a big deal - but the second part is where I'm confused. I can't even come up with a name for it. First I thought magnitude, but I don't really see how that works. My calculus is a little rusty, so bear with me while I ask this question. What is it? How did we get from equation #2 to #3?

    Thanks
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 17, 2009 #2

    gabbagabbahey

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    It's the magnitude of the vector in #2.

    [tex]\left|\frac{d\textbf{r}}{d\theta}\right|=\left(\frac{d\textbf{r}}{d\theta}\cdot\frac{d\textbf{r}}{d\theta}\right)^{1/2}[/tex]
     
  4. Sep 17, 2009 #3
    Thanks! I suppose what threw me off magnitude was the trig functions; they're looking funny when you first do them but they magically boil down to something more manageable.
     
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