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Vector basic problem

  1. Oct 9, 2017 #21

    Orodruin

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    When you insert a bold letter you are in actuality insertin BBcode into your LaTeX expression. If you want to make something bold in LaTeX, you must use LaTeX commands. But in general there is no need to make a vector bold and to have a vector arrow.
     
  2. Oct 9, 2017 #22
    The question is as follows: Find the position vector of the midpoint AB.
     
  3. Oct 9, 2017 #23

    FactChecker

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    Aha! Yes, that is different and it is well defined. The midpoint of the line from point A to point B is at (A+B)/2 = ( (3+1)/2, (2+3)/2, (5+2)/2 ) = ( 2, 2.5, 3.5 ). And the vector to that point is based at (0,0,0) and denoted by ##\vec{( 2, 2.5, 3.5 )}##.

    You can also get it by taking your vector of the OP, ##\vec{AB}##, and adding it to the position vector of point A.
    ##\vec A + \vec {AB} = \vec{(3,2,5)} + \vec{(-1,0.5,-1.5)} = \vec{(2,2.5,3.5)}##
     
    Last edited: Oct 9, 2017
  4. Oct 9, 2017 #24

    FactChecker

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    Thanks. I also tried \textbf{}, which didn't work. Maybe I did it wrong. Anyway, I agree that it was not needed and I stopped trying.
     
  5. Oct 9, 2017 #25

    Orodruin

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    If I have to put bold stuff in equations, I usually use \boldsymbol instead. I think it looks much better as it uses an italic math font instead of text font - also it works for Greek letters. Here is the difference compared to using \textbf or {\bf }:
    $$
    \boldsymbol{a}\cdot\boldsymbol{\mu} = \textbf{a}\cdot \textbf{\mu} = {\bf a}\cdot{\bf \mu}
    $$
    Either way, I much prefer using \vec as it looks more similar to what you would write by hand and I think it is just confusing for students that we write things one way in typeset text and another on the blackboard.
     
  6. Oct 9, 2017 #26

    Ray Vickson

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    The commands "\bf{x}" or "\mathbf{x}" both produce ##\bf{x}## and ##\mathbf{x}##. The commands "\mathbf{x}^2" and "\mathbf{x^2}" produce ##\mathbf{x}^2## and ##\mathbf{x^2}##, one of which has a bold superscript and the other not. The command \mathbf{x \times \alpha}" produces ##\mathbf{x \times \alpha}## (with bold ##\bf x## but non-bold ##\alpha## and non-bold ##\times##), while "\boldsymbol{x \times \alpha}" produces ##\boldsymbol{x \times \alpha}##, where everything is in bold.
     
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