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Homework Help: Velecity Vector

  1. Feb 12, 2007 #1
    1. The problem statement, all variables and given/known data

    If http://net.dinhweb.de/vector.gif [Broken],[/URL] where b and c are positive constants, when does the velocity vector make an angle of 45.0 with the x- and y-axes?

    2. Relevant equations

    I'm not sure.. I've got a million equations but none of them seem to fit the problem the way I need. I know this isn't relevant to the question, but the prompt asks for "t=?"

    3. The attempt at a solution

    I've got this thing worked down to b divided by c (b/c), but apparently my "answer is off by a multiplicative factor." I've already taken a few shots at guessing the coefficient I need, but no luck there, so I decided to give this a try.

    I didn't think any kind of "multiplicative factor" was necessary because equal "i hat" and "j hat" values would give a 45 degree angle. Apparently not and here I am stuck on this problem

    Any kind of help would be greatly appreciated =]
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 12, 2007 #2


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    Do you know how to use the scalar product to determine the angle between two vectors? i.e.[itex]\bold{x}\cdot \bold{y}=xy\cos\theta[/itex] where [itex]\theta[/itex] is the angle between the two vectors.
  4. Feb 12, 2007 #3
    Somewhat.. Researching the answer earlier through my textbook actually led me to the exact page with the scalar product definition, but I couldn't figure out how it tied into the problem.

    It's actually a couple chapters back from the original problem, but now I'm in the process of reading it over again.
    Last edited: Feb 12, 2007
  5. Feb 12, 2007 #4


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    Well, try looking at r.i and r.j, since you know the angle that r must make with the x and y axes.
  6. Feb 12, 2007 #5
    Hm.. I think I understand a little better now..

    So I have the component direction and magnitude, but finding the dot product of r and i or r and j gives me the magnitude of r which is the component of either i or j? And that magnitude is the factor I'm missing?
  7. Feb 12, 2007 #6
    I have looked at this, and plugged b/c into excel and it works for 5 different b and c constants. Thus for me, b/c is correct.
  8. Feb 13, 2007 #7
    Well I feel stupid..

    Someone hinted to "find the derivative of the positive vector".. Which meant next to nothing to me, but I decided to give it a try anyway and use the coefficients I would've gotten if I took the derivative of the whole equation. It turns out the "mutiplicative factors" I was missing was a 2 and 3, so the answer was..

    t = (2b)/(3c)

    I don't know if it's just me, but that answer doesn't make any sense and now I'm more confused than ever.
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