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Homework Help: Find the angle between A and B

  1. Jun 18, 2009 #1
    Three coplanar forces - A, B and C - have magnitudes of 1000N, 1200N and 1700N respectively. They act on a body such that the resultant force is zero. Find the angle between A and B.

    Well, I know that you can create your own axis from one of the forces. However, the solution that my teacher wrote up confuses me. They assumed one of the forces from the x axis and one of the forces started on the y axis. I have no idea why they did that because I always thought you could only assume the position of one of the forces on the X axis and work from there.

    However, if anyone could clarify this for me, that would be helpful. Thanks in advance.
  2. jcsd
  3. Jun 18, 2009 #2


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    Re: Vectors

    Yes, I think you have insufficient information. For example, if the angle between A and C were given (for example as being a right angle, such that you can put C along the y-axis) then the question can be solved. Otherwise you can draw C in any direction you want, so as to make the angle between A and B equal to anything you want.
  4. Jun 18, 2009 #3
    Re: Vectors

    Incorrect. There is sufficient information to solve the problem, but if the teacher defined two of the vectors as orthogonal to each-other, he made a mistake as well, a big mistake at that, since if the triangle did in fact have a right angle, the statement 10²+12²=17² would have to be true, which it isn't.

    Remember, if you are given the length of all 3 sides of a triangle, you know EXACTLY what it looks like.

    Another hint:
    The net force is 0, so the vector sum of the three vectors is 0, meaning 0 translation = a close shape, a triangle.
    The final answer I got was ~100.8°
    Last edited: Jun 18, 2009
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