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Homework Help: Relative Velocity Question

  1. Sep 25, 2009 #1
    1. The problem statement, all variables and given/known data
    Here is the problem.


    2. Relevant equations

    Va/b = Va - Vb

    3. The attempt at a solution
    s - swimmer
    w - water

    Vw = 0.3m/s
    Vs/w = 2.5m/s

    therefore, Vs - Vw = 2.5m/s
    Vs - 0.3m/s = 2.5m/s
    Vs = 2.8m/s

    Now I am a bit confused at this point. It is asking which direction the swimmer needs to swim. What I have tried to do is draw 1 velocity vector pointing directly upward (the 0.3m/s one) and another pointing 38.66 degrees from the horizontal (I calculated that angle using the dimensions given). Am I on the right course to solving the problem, or am I completely wrong?
  2. jcsd
  3. Sep 25, 2009 #2


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    Hi Oblivion77! :smile:
    Nooo …you can't say 2.5 + 0.3 = 2.8 …

    relative velocities are vectors, so they add (and subtract) like vectors.
    Yes that's fine so far …

    now complete the vector triangle …

    you know the length and direction of one side, the angle of another side, and the length of the third side …

    from that, you should be able to find the other angle(s). :smile:
  4. Sep 25, 2009 #3
    Thanks for the response. I was wondering If I have this diagram right?


    If it's correct would I just then need to solve for the angles? Sorry the drawing isn't really to scale.
  5. Sep 26, 2009 #4


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    HI Oblivion77! :smile:

    (just got up … :zzz:)
    Yes, the diagram is fine (and the arrows are the right way) …

    so you know that Vs has length 0.3, and Vw - Vs has length 2.5, and you know one of the angles …

    now use the sine rule and the cosine rule. :smile:
  6. Sep 26, 2009 #5
    Thanks for the response, doesn't Vw have a length of 0.3 and Vs - Vw = 2.5?
  7. Sep 26, 2009 #6


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    oops! yes, I got them the wrong way round. :redface:
  8. Sep 26, 2009 #7
    Re: oops!

    No problem, thanks for the help.
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