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Finding the resultant of two vector of an oblique triangle

  1. Oct 7, 2011 #1
    1. The problem statement, all variables and given/known data

    A car travels 20.0 km due north and then 35.0 km in a direction 60.0° west of north. Using a graph, find the magnitude and direction of a single vector that gives the net effect of the car's trip. This vector is called the car's resultant displacement.

    I am studying for a physics test, and this is a problem out of my textbook. The author gives the answer of the resultant to be 48 km, and the angle β, the angle between vector A and the resultant, to be 39° west of north.

    2. Relevant equations

    From the book, because I missed class the day we went over this, I have learned that to find the resultant one simply adds vector A to vector B.

    R= A + B

    Also, from different physic web sites, some people have suggested the pythagorean theorem; however, I believe that only works with right triangles, not oblique.

    R2= A2 + B2

    Lastly, I came across this equation.

    c2= a2 + b2 - 2abcos(c)

    3. The attempt at a solution

    I have tried all 3 equations.

    1) R= 20 + 35= 55

    2) R2=202 +352
    R2= 1625
    R= (sqrt 1625)
    R= 40.3112887

    3) R2= 202 + 352 - 2(20)(35)cos(60)
    R2= 1625-1400cos(60)
    R2= 691.29
    R= 26.2923943

    Obviously, none of these solutions match the one given by the author. How am I supposed to work this problem to find the right solution? As far as finding angle β, I have no idea where to start
  2. jcsd
  3. Oct 7, 2011 #2


    User Avatar

    Staff: Mentor

    Add vectors by adding their components. The result is the components of, you guessed it, the resultant! So first break down your given vectors into their components.

    The two components of your resultant will be at right angles to each other, just as in a Pythagorean triangle. So use the appropriate formula to find the magnitude of that vector from its components.

    Also, look at your graph of the setup to determine how you might find the requested angle of the resultant.
  4. Oct 10, 2011 #3
    Thank you for your help, but could you explain how I am to find vector components?
  5. Oct 10, 2011 #4


    User Avatar

    Staff: Mentor

    Draw a diagram of a vector on a set of Cartesian coordinates. The components of the vector are the perpendicular projections of the vector on the coordinate axes.

    If the vector projects from the origin to a point (x,y), then x and y are also the magnitudes of the components. Clearly, then, given a vector of length R that makes some angle θ with the x-axis, the components can be found using the basic trig formulas for a right triangle.
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