Cosine Rule , what good is it ?

[junior_J]

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 ??

 

[Zelos]

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

 

[courtrigrad]

the 60 degrees is negative, because it is east of north.

 

[junior_J]

negative displacement ? would that apply to the cosine rule ?

 

[HallsofIvy]

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.

 

[junior_J]

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 ?

 

[courtrigrad]

yes you would, because you only care about magnitude. The direction comes later.