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Net force from a straight-line resultant?

  1. Oct 27, 2012 #1
    1.Two concurrent forces act on a box. The first force acts east at 13 newtons and the second force acts west at 17 newtons. What is the net force acting on the box?

    >Draw each vector using the tip to tail method.
    >Draw your resultant and label it "R"
    >With your ruler determine the magnitude of the resultant. Using a protractor determine the direction of the resulting force. The resultant is the Net Force acting on an object.

    2. Equation Used: c^2=a^2=b^2

    3.I am completely confused as to what my teacher expects here. He gave no hints on how to do this problem nor did he teach finding a resultant on a straight line. From my understanding, one vector going east and one vector going west makes a straight line. I have no clue how to draw the diagram, so I mathematically, using the Pythagorean Theorem, determined that the resultant would be 21.4 newtons, However, I have no clue on how to draw these two vectors from tip to tail, where and how to draw the resultant, and how to get the direction of the resultant be it East or West.
     
  2. jcsd
  3. Oct 27, 2012 #2

    CWatters

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    I can't see a way to explain it without giving the whole answer away so...

    Draw an arrow/vector 13cm long pointing east. Then from the head of that arrow/vector draw the second 17cm long pointing west (eg over the top of the first if necessary). The resulting vector is from the tail of the first to the head of the second. Should be 4cm long pointing west.

    See also..
    http://www.physicsclassroom.com/class/vectors/u3l1b.cfm
     
  4. Oct 27, 2012 #3

    PeterO

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    If you consider an equivalent problem, using displacement vectors rather than Force vectors you might get an understanding.

    In your problem, Force plus Force gives net force.
    In the parallel problem, displacement plus displacement gives net displacement.

    If you walk 13 km East, then 17 km West, what is you net displacement?

    EDIT: You would only use Pythagoras if one of the forces was acting at right angles to the other.
     
  5. Oct 28, 2012 #4
    Yes, but I haven't been taught displacement vectors yet, or any other formula for dealing with this problem.
     
  6. Oct 28, 2012 #5
    I tried that, but shouldn't the resultant be the longest? I drew the resultant from the head of 17n past the tail in order to get proper scale for 21.4 n. My scale was 1cm=2n,
     
  7. Oct 28, 2012 #6

    PeterO

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    never mind being taught displacement vectors - and I will reduce the scale.

    Suppose you were to do the following:

    Stand on 3rd base of a baseball diamond - with a compass in your hand so you know where the directions are.

    Walk 13m due East, STOP, then walk 17m West.

    Where would you be - relative to 3rd base?
     
  8. Oct 28, 2012 #7

    PeterO

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    Why should the resultant be the longest.

    Let A = +6 and B = -5

    What is (A + B) =

    Note: A + B is the answer, or resultant. Is it the biggest value here?
     
  9. Oct 28, 2012 #8

    CWatters

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    No not allways. Imagine two equal forces acting in opposite directions. The result is zero force.
     
  10. Oct 28, 2012 #9
    thanks so much everyone, I think i figured it out now!
     
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