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Homework Help: A simple question, but I keep getting it wrong!

  1. Jul 6, 2007 #1
    Hi this seems like a simple question (and probably is) but I keep getting the answers wrong.

    1. The problem statement, all variables and given/known data

    Two ships, A and B, leave port P at the same time. Ship A travels due north at a steady speed of 15kmh[tex]^{-1}[/tex] and ship B travels N 60[tex]^{o}[/tex] E at a speed of 10kmh[tex]^{-1}[/tex].

    i) What is the distance and direction from A to B after 1 hour?
    ii) What is the velocity of B relative to A?

    2. Relevant equations

    c[tex]^{2}[/tex] = a[tex]^{2}[/tex] + [tex]^{2}[/tex] - 2.a.b.COSC

    COS 60[tex]^{o}[/tex] = 0.5; SIN 30[tex]^{o}[/tex] = 0.5; cos 30[tex]^{o}[/tex] = ([tex]\sqrt{3}[/tex])/2

    3. The attempt at a solution

    The workings can be seen here (sorry I prefer to write):
    http://www.unicyclist.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=551574&g2_serialNumber=1" [Broken]

    The answer given in the book for the distance between A and B in i) should be 13.33Km, my calculations make it 13.23Km, it's close but not enough for me to be certain I've done it correctly.

    The direction for i) is correct they've (in my opinion) written it a strange way.

    I wrote 139.11[tex]^{o}[/tex] (from North clockwise) or E 49.11[tex]^{o}[/tex] S. They wrote S 40[tex]^{o}[/tex] 54' E - this means S 40.9[tex]^{o}[/tex] E

    My answer for ii) is very close 13.23kmh[tex]^{-1}[/tex], but they give 13.22kmh[tex]^{-1}[/tex].

    Can anyone shed any light on what I'm doing wrong?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 6, 2007 #2

    Doc Al

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    Staff: Mentor

    Looks good to me. Your answers are correct.
  4. Jul 6, 2007 #3
    Would you say then that the book's answers are incorrect?

    Maybe they've used a different method and rounded with the cos30 rather than using it's surd value?
  5. Jul 6, 2007 #4

    Doc Al

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    Staff: Mentor

    I agree with your answers; where the book disagrees, the book is incorrect. Beats me why--but it does happen.
  6. Jul 6, 2007 #5
    okay, thank-you for taking the time to look at my messy workings.
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