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Wrong answer in textbook

  1. Jun 18, 2014 #1
    1. The problem statement, all variables and given/known data
    (Problem about a collision) (...). What then is their velocity in (a) unit-vector notation and as (b) magnitude and (c) angle.

    I will not post the problem due to copyright issues and that it is absolutely pointless now.

    3. The attempt at a solution
    I was able to solve (a) and (b).
    However, my answer for (c) differs from that of the textbook.
    [tex]\text{a)} \;(2.67 \;\text{m/s})\hat{i}+(-3.00 \;\text{m/s})\hat{j}[/tex]
    [tex]\text{b)} \;4.01 \;\text{m/s}[/tex]
    [tex]\text{c)} \;\theta{} = \tan^{-1}\left(-\frac{3.00}{2.67}\right)=-0.845397=-48.4^{\circ{}}[/tex]

    While the book provides +48.4° as the angle.

    Am I correct? You can solve (c) from (a) and (b) [my answers are identical to those of the book].

    It is from Fundamentals of Physics (Extended) - 9th ed.
  2. jcsd
  3. Jun 18, 2014 #2
    Well, without thinking too much about it, the vector given in (a) is in the fourth quadrant. The angle 48.4° is in the first quadrant. Your answer, however, is definitely in the fourth quadrant.
  4. Jun 18, 2014 #3


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    You are correct. The text should not have stated the answer as simply 48.4o.
    If they had stated it as 48.4o below the positive x direction, then that would have been a good answer. But they didn't.
  5. Jun 18, 2014 #4
    Yes, that's what I thought. I was pretty sure I was right, but as this book has presented very few mistakes already I thought I could be missing something.

    Thank you guys for looking at it.
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