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Simple vector addition question

  1. Apr 5, 2006 #1
    just a quick question, i've never dealt with vectors before, they say you are simply supposed to add them, the same way you would add any two numbers, ie: a + b = c
    yet whenever they add two vectors, they end up with a different sum..
    ie: 1m + 2.5m = 7m
    this seems so simple, what am I missing? :redface:
     
  2. jcsd
  3. Apr 6, 2006 #2
    Suppose you go 3m east then turn and go 4m north. How far from your starting point are you? 7m ? Or 5m ?

    Suppose you go 3 m east then turn and go 3 m west. How far from your starting point are you? 6m ? Or 0m ?

    The essential thing about a vector is that it points in a particular direction and you have to take account of this direction when adding vectors.
     
  4. Apr 6, 2006 #3

    arildno

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    christina:
    You are probably misunderstanding the addition principle of vectors to mean you can add the lengths of two vectors to get the length of the new one.
    This is untrue!
     
  5. Apr 7, 2006 #4
    thanx for both your responses.

    rdt2, i understand the second question, the final displacement, if u go 3m east than 3m west, you are back in the same place, making the sum o
    if u add them together its (-3) + (3) leading to 0.

    but if u go 3m (east) and 4m (north) , how do you get the sum to total up to 5m?
    aren't north and east positive numbers?


    also if i am not supposed to be adding the vector lengths (m) what am i supposed to be adding?
     
  6. Apr 7, 2006 #5

    Hootenanny

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    Try drawing out the paths. Draw a dot as your starting point. Then draw a line horizontally east of length 3cm. Draw another dot at the end of this line. From this point, draw a vertical line north of length 4cm, place a dot at the end of this line. Now draw a straight line from your finnishing point to your starting point. How long is this line?

    -Hoot:smile:
     
  7. Apr 7, 2006 #6

    BobG

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    Maybe this will give you a feel for what you're trying to do.

    Vector Calculator

    Mathematically, you want to break each vector down into Cartesian coordinates (x and y components). Add the x's together; add the y's together; and you have your result in Caresian coordinates. To get the magnitude of the resulting vector, plug the new x and y components into the Pythagorean Theorem.
     
  8. Apr 9, 2006 #7
    hootenanny: i still have absolutely no idea how you can get 5, from adding 3 and 4. if you draw a line connecting the beginning to the end, i just don't get it! i am definitely missing something. however i understand how you could get five if you use the pythagoream theorum (sp) to solve for the hypotenuse..

    i feel like the solution to my question is so simple, and i am just missing some vital information, or rules.. i never took algebra, geometry, trigonometry, or any physics classes before this grade 12 physics that i am taking right now, so i am unaware of most of the concepts.
     
  9. Apr 9, 2006 #8

    Hootenanny

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    Drawing out the triangle (what I told you to do) is the same as using pythag. With vector you have to consider their direction in addition to their magnitude.
     
  10. Apr 9, 2006 #9

    Hootenanny

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  11. Apr 10, 2006 #10
    i read the webpage, it was helpful thanx.
     
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