1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Velecity Vector

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

    If [​IMG], 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: Feb 12, 2007
  2. jcsd
  3. Feb 12, 2007 #2

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    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

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Velecity Vector
Loading...