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Homework Help: Vector question, concerning direction

  1. Jan 11, 2007 #1
    1. The problem statement, all variables and given/known data
    2 vectors of equal magnitude act in a vertical plane perpendicular to each other. If their resultant is 81.0 N directly downward then
    Part A use components to find thew magnitude of each of the two vectors
    and Part B use components to find the angle each vector makes with the resultant force, use counterclockwise angles vectors make with downward direction.

    2. Relevant equations
    No equations given. I am not quite sure how to start, since no magnitudes are even given, and only one angle which I do not understand.

    3. The attempt at a solution
    I am stumped by the language, what does 81 N directly down even mean? Does that mean that the resultant faces at an angle 81 degrees off the x axis to the east, west, or am I way off? Sorry, this is my first week of my first Physics course ever, I know nothing!!
  2. jcsd
  3. Jan 11, 2007 #2

    Chi Meson

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    "81 N" : That's the "magnitude." Magnitude is a word that means "size" or "quantity." 81 N directly down means 81 newtons of force, pointing directly down. Directly down is the direction. OK so far?
  4. Jan 11, 2007 #3


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    I believe that the "N" stands for newtons, the magnitude of the force vector. So that means that an 81 newton force acts vertically down. The sum of the vertical components of each of the unknown (but equal in magnitude, and perpendicular to each other) vectors must add to 81, because that is the resultant force in the downward vertical direction. The sum of their horizontal components must add to 0, because there is no horizontal resultant force, since the resultant force acts directly vertically down. Does this help at all?
  5. Jan 11, 2007 #4
    Okay, so when I draw this bad boy up, my resultant points directly down, but the other two vectors form a right angle and everything connects tail to head or whatnot. Now, if 81 N is the only number I am given, how can I find the magnitude of the two vectors? Will they be equal? Essentially, I don't know the set-up for the problem, once I know how to set it up, I am sure I can get it. Thanks for the help so far, love that their is a forum out there for this.
  6. Jan 11, 2007 #5
    You have two vectors at a right angle to each other, and one connecting them, pointing straight down. Just draw it and solve for the right triangle.
  7. Jan 11, 2007 #6


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    Your life would be extremely easy if you'd set your problem up in a coordinate system and if you'd choose the standard basis {i, j} and express your vectors in that basis. If you do not know what I'm talking about, then all you need to do is recall what a component of a vector is and use basic geometry/trigonometry.
  8. Jan 11, 2007 #7
    Okay, so essentially I have a 90, 45, and 45 degree angle, then does this talk of "use clockwise angles vectors make with downward direction" even matter?
  9. Jan 11, 2007 #8
    Sorry guys, I am hopeless, I am going to try a few things then I will get back to you if I am still lost.
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