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Cosine Rule , what good is it ?

  1. Oct 16, 2006 #1
    Cosine Rule , what good is it ????

    Does the Cosine rule hold true for say negative lengths ??? as in a vector quantity like displacement ???

    I came across this problem which had -15km and 10km as the known sides whereas the angle opposite the unkown side is 60 degree ....

    I tries using c^2 = a^2 + b^2 - 2*a*b cosC but this didnt work .... then on account of frustration i forcibly changed the '-' part of the formula to a '+' only to find that it works ... ???

    Can someone explain this to me ??
  2. jcsd
  3. Oct 16, 2006 #2
    ther is no such thing as negative lenght, there is negative coordinates since somepoint has to be 0 and one side will be posetive and the other negative
  4. Oct 16, 2006 #3
    the 60 degrees is negative, because it is east of north.
  5. Oct 16, 2006 #4
    negative displacement ????? would that apply to the cosine rule ?
  6. Oct 16, 2006 #5


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    There is no such thing as a negative length. A vector quantity (say, in a one-dimensional problem on a number line), or the components of the vector might be negative but the length is always non-negative. If you are using the cosine law in a vector problem, use the lengths, not the components.
  7. Oct 16, 2006 #6
    so if i had a velocity oh say -15 km/hr ..... id use 15 instead of -15 ?? as one of the sides of a triangle ???
  8. Oct 16, 2006 #7
    yes you would, because you only care about magnitude. The direction comes later.
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