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Homework Help: Simple Vector prob

  1. Feb 4, 2013 #1
    1. The problem statement, all variables and given/known data

    The current in a river is flowing northwest with a speed of 1.5 m/s. You are swimming due east with a speed of 2 m/s relative to the water. What is your speed (in m/s) relative to the ground?

    2. Relevant equations
    vm,g = vw,g + vw,m

    3. The attempt at a solution

    I have tried all sorts of playing around getting answers such as .5, 2.5, 3. I set x to the east as positive and y north positive. The water is actually traveling in a negative x direction so how do I account for that?
    Last edited: Feb 4, 2013
  2. jcsd
  3. Feb 4, 2013 #2


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    By using minus signs :smile:
    Pls post your working.
  4. Feb 5, 2013 #3
    I dont know if I'd call it work but i just plugged in the values that were given in the the problem into the equation stated. The different answers come from different combos of these numbers and then i checked them against an online quiz. Something tells me i have to turn these into a parametric equation but im at a loss
  5. Feb 5, 2013 #4


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    Please show your work! You say "i just plugged in the values that were given in the the problem into the equation stated." I suspect that the problem is that you do not understand the equation. Do you understand that [itex]v_{wg}[/itex] and [itex]v_{mm}[itex] are vectors, not numbers?
  6. Feb 5, 2013 #5
    I will show my work. I apologize....I was posting from my phone on the bus ride home last night and this morning.

    My next thought it to try and deconstruct -1.5 m/s NW into i and j. Using θ=45 in quad 2 for NW I tried to solve -1sin (x/-1.5) = 45 and came up with -1.062. Then set 1.5 = ((1.062)2 + (x)2)1/2 and solved x = 1.368267.

    Vw,g = -1.062i + 1.368267j
    Vm,w = 2i

    Vm,g = Vw,g + Vm,w = -1.062i + 1.368267j + 2i

    = .938i + 1.368267j

    (.9382+1.3682672)1/2 = 1.67 m/s

    I'm sure I'm all over the place
  7. Feb 5, 2013 #6
    I found it. Just going through the motions of showing the work helps tremendously. Thanks folks
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