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Homework Help: Moment about the origin

  1. Apr 24, 2010 #1
    1. The problem statement, all variables and given/known data
    Find the moment about the origin of a vector of magnitude 100 units acting from
    A=(2, 0, 4) to B=(5, 1, 1). What is the moment about the point C=(−1, 2, 1)?

    2. Relevant equations
    M= r x F

    3. The attempt at a solution
    I did the quesion, but I'm not sure if it's correct. Can anyone check it for me?

    Attached Files:

  2. jcsd
  3. Apr 24, 2010 #2


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    The "moment of a vector about a point" is the magnitude of the vector times the length of a line segment from the given point perpendicular to the line of the vector.

    Since the vector acts "from (2, 0, 4) to (5, 1, 1)", its line of direction is given by x= 3t+ 2, y= t, z= -3t+ 4. You need to find the distance from (0, 0, 0) to that line and from (-1, 2, 1) to that line . Then multiply those by 100.
  4. Apr 24, 2010 #3
    So I need to find |OL| where O = (0, 0, 0) and L = (2+3t, t, 4-3t), thus

    Making a moment of

    Simillary to Q = (-1, 2, 1)

    Attached Files:

    • Mol.jpg
      File size:
      5.4 KB
    • OL.jpg
      File size:
      4.7 KB
    • QL Mql.jpg
      QL Mql.jpg
      File size:
      11.9 KB
  5. Apr 26, 2010 #4
    Ahhh come on guys help me out here! Any hints?
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