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Vectorial Equation

  1. Oct 4, 2012 #1
    Hi everyone,

    I was wondering, if I have this vetorial equation:[tex]\vec{F}\cdot\vec{v}=\vec{a}\cdot\vec{v}[/tex]
    I can say that [tex]F\cos (\theta)=a\cos (\phi),[/tex]where [itex]\theta[/itex] and [itex]\phi[/itex] are the angles that [itex]\vec{F}[/itex] and [itex]\vec{a}[/itex] do with the vector [itex]\vec{v}[/itex], respectively.

    The doubt is: does [itex]Sign(\cos (\theta))=Sign(\cos (\phi))[/itex]? Because [itex]F[/itex] and [itex]a[/itex] represent the modulus so have to be positive, right?

    Thank you
    Last edited: Oct 4, 2012
  2. jcsd
  3. Oct 4, 2012 #2


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    Science Advisor

    I think it's
    [tex]F\cos (\theta)=a\cos (\phi),[/tex]
    I'm not sure where the sin came from.
  4. Oct 4, 2012 #3
    Yeh it's [itex]\cos[/itex], I was distracted. I'm going to change it. Thank you.
    Last edited: Oct 5, 2012
  5. Oct 4, 2012 #4
    If the equality is true, then the signs of the cosines must be the same.
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