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Question on Arc Length parameterization.

  1. Apr 8, 2013 #1
    This is an example in book by Howard Anton:

    Vector form of line is ##\vec r=\vec r_0+t\vec v## where ##\vec v## is parallel with the line. So both ##\vec r## and ##\vec r_0## are POSITION VECTORS.

    To change parameters,
    1)Let u=t ##\Rightarrow\; \vec r=\vec r_0+u\vec v##.

    2) ##\frac {d\vec r}{du}=\vec v\;\Rightarrow\;|\frac {d\vec r}{du}|=|\vec v|##

    ##s=\int_0^t |\frac {d\vec r}{du}|du=t|\vec v|\;\Rightarrow\; t=\frac{s}{|\vec v|}##

    3)##\Rightarrow\; \vec r=\vec r_0+\frac{s}{|\vec v|}\vec v##



    My question is in #2 above. In order for ##\frac {d\vec r}{du}=\vec v## which is the tangent vector of the curve traced by ##\vec r ##, ##\vec r ## has to be a VECTOR VALUE FUNCTION, NOT JUST A POSITION VECTOR. This means ##\vec r =\vec r(w)## where w is the independent variable that make the tip of ##\vec r## tracing out the line when w increases or decreases.( of cause it can be a vector value function of many variables also).

    As you see, my problem is there are TWO parameters, t and w. The book only change parameter of t, which has nothing to do with the vector value function ##\vec r(w)##. t only tell the line is multiple of ##\vec v##. In another word, this example totally miss the point in changing parameter. The parameter needed to be change is w, not t.

    Please comment on this.
     
    Last edited: Apr 9, 2013
  2. jcsd
  3. Apr 9, 2013 #2
    Further more, the next example is to find arc length parametrization of the line where ##x=2t+1,\;y=3t-2## with ##\vec r_0=<1,-2>## and parallel to vector ##\vec v=\hat x 2+\hat y 3##

    ##\Rightarrow \vec r=\vec r_0+s\frac{\vec v}{|\vec v|}=(\hat x-\hat y 2)+s\frac{\hat x 2 +\hat y 3}{\sqrt{13}}##

    Thereby after change of parameter:

    ##x=\frac {2}{\sqrt{13}}s+1,\;y=\frac {3}{\sqrt{13}}s-2##



    This example use the same variable t as the parameter for x and y. Can you even do that? As I explained in the last post, there should be two independent variables w and t, not just t alone. Please comment on this also. I am confused.
     
  4. Apr 9, 2013 #3

    SteamKing

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    The components of a vector can be expressed in terms of some parameter t. In post 2 above, think of the x and y as being the number of unit vectors i and j combined, according to values for the third parameter t.

    If the vector R(t) = [x(t), y(t)], then as t runs from 0 to 2, R(0) = [1,-2] thru R(2) = [5,4] and all values in between, consistent with the definition of x and y in terms of t.

    Essentially, R could be defined as R(t) = [(2t+1),(3t-2)] instead.
     
  5. Apr 9, 2013 #4
    Thanks for the response, I also work it out while I was waiting. Thanks.
     
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