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!

Relative Velocity Question

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

    2ivd09j.png

    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

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    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?

    2502zrl.png

    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

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    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

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    oops!

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

    No problem, thanks for the help.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook