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Simple vectors

  1. Jan 2, 2008 #1
    adding vectors and getting their resultants are Quite easy but understanding the problem is not.. in my problem it says that three of the forces are 100N, 60 degrees above the x axis; 200N, 140 degrees above the x axis and 250N, 320 degrees above the x axis..The resultant is 100N along the negative y axis.

    Im confused on what does above the x axis mean..does it mean im going to start from quadrant I which is above the x axis or the degree should stop or end above the x axis.
  2. jcsd
  3. Jan 2, 2008 #2

    Shooting Star

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    It is the angle the line of action of the vector makes with the x-axis, measured in the anticlockwise direction from the x-axis to the vector. E.g., a vector pointing toward the negative y-axis makes an angle of 270 deg with the x-axis.
  4. Jan 2, 2008 #3


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    The phrase "320 degrees above the x-axis" is a little peculiar! As Shooting star said, angles in a coordinate system are measure "anticlockwise" (I would say "counterclockwise") from the positive x-axis. The positive y-axis is 90 degrees from the positive x-axis, the negative x-axis is 180 degrees from the positive x-axis and the negative y-axis is 270 degrees from the positive x-axis. Since 270< 320< 360, the end of that vector is actually below the x-axis, in the fourth quadrant.
  5. Jan 2, 2008 #4


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    Now that you know what "above the x-axis" means, you might also notice that something else is peculiar (unless you haven't stated the whole problem). You've given 3 forces and also the resultant force. However if you only take into account the 3 force vectors, you won't obtain a resultant force of 100N along the negative y-axis.

    Just curious; does your question ask, what additional force vector is needed to obtain the resultant?

    (i solved analytical, by hand as well as by computer, also solved graphically and confirmed the contributions of the 3 given force vectors do not combine to the given resultant).
  6. Jan 2, 2008 #5
    Yes it does..the question would be find the fourth force..

    Thanks a lot guys!
  7. Jan 2, 2008 #6
    Yah..so what do you think above the x axis mean..should the angle end above the x axis or the turn start at the positive x axis as it usually does..
  8. Jan 3, 2008 #7

    Shooting Star

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    The turn starts at the +ve x-axis and then you go CCW until you catch up with that vector.
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