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Help with Displacement ( Vectors)

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A Hiker travels in the following directions
    i) 3.2 Km ( N 42 W)
    ii) 2.1 Km ( W 12 S )
    iii) 1.9 Km ( E 62 N)

    A) what is the resulting displacement?



    2. Relevant equations



    3. The attempt at a solution
    A)
    X Dir

    3.2 sin 42 = 2.14
    2.1 cos 12 = 2.05
    1.9 cos 62 = .89
    total = .94 <<<<<, Total is wrong :(
    Y Dir

    3.2 Cos 42 = 2.38
    2.1 sin 12 = .44
    1.9 sin 62 = 1.68
    .26 <<< Total is wrong too

    Can anyone explain on the X and Y Directions, Like where to Use positive and Negative Sign. I really Need help with this :P Beacuse it confuses me that were should i use the signs other than that rest is perfect.

    Thanks for the help In Advance ^.^
     
  2. jcsd
  3. Jan 25, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    Welcome to PF.

    The way I do it is draw a diagram.

    I set N as positive Y and East as positive X.

    Then I use that to keep track of the sign.

    The N 42 W I read in quadrant notation in this regime then as +Y, -X with the angle made with the vertical Y axis.

    Fwiw, I think quadrant notation usually uses just N or S followed by angle or direction E or W. For some reason this problem is using E and W as well, but I still read it as Direction - angle - direction off that direction.
     
  4. Jan 25, 2009 #3

    LowlyPion

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    Homework Helper

    I would assume that the problem is using this convention as well, unless you have previous experience with the conventions used in your course work.
     
  5. Jan 25, 2009 #4
    thanks for the help, but ..... i used it like u said still wrong... I have the answer key with me so yeah here's the problem aagain :

    3.2 sin 42 = +2.14
    2.1 cos 12 = -2.05
    1.9 cos 62 = +.89
    total = .94 <<<<<, Total is wrong :(
    Y Dir

    3.2 Cos 42 = - 2.38
    2.1 sin 12 = -.44
    1.9 sin 62 = +1.68
    .26 <<< Total is wrong too


    HEre's the Answer key i have it with me :P

    3.2 sin 42 = +2.14
    2.1 cos 12 = +2.05
    1.9 cos 62 = -.89
    total = 3.304
    Y Dir

    3.2 Cos 42 = + 2.38
    2.1 sin 12 = - .44
    1.9 sin 62 = +1.68
    Total ==== 3.62

    Meh can u Look again xD if u need more details i'll scan and post it on the web :P
     
  6. Jan 25, 2009 #5

    LowlyPion

    User Avatar
    Homework Helper

    It looks like that would be the problem.

    When they say N 42 W, they are meaning 42 N of West

    It seems they chose West as positive too.

    Draw it out carefully, only reading it as:
    North 42 degrees from West
    West 12 degrees from South.
    East 62 degrees from North.

    I think that yields the right answer.

    To answer your original question so long as you are consistent in staying with whatever sign you choose for your coordinates you should get to the same result.

    However understanding their notation is also key.

    Good Luck.
     
  7. Jan 25, 2009 #6
    ok thanks :P btw my exam on physics is next week so i was having some trouble with that :P
     
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