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Relative motion help please

  1. Oct 26, 2014 #1
    • OP warned about not including efforts to solve the problem
    1. The problem statement, all variables and given/known data
    A ships captain wishes to sail his ship north -east. A current is moving his ship with a velocity of 5.0km/h . If the ship has a maximum speed of 30 km/h what is the ships required heading?

    2. Relevant equations
    Cos angle=A/H

    3. The attempt at a solution
    Heading=cosine (5/30)
    Heading=80 degrees
    The answer is [N 38 degrees E]
     
  2. jcsd
  3. Oct 26, 2014 #2
    Just out of curiosity, are you given the direction in which the current is moving his ship?
    Edit: maybe I'm mistaken, but I feel like you need that information to solve this problem..
     
  4. Oct 26, 2014 #3
    Yes it says south.
     
  5. Oct 26, 2014 #4
    Ok, for problems like these it is essential to draw a diagram. You are trying to find the angle the ship is making with the x-axis (east axis). First you will need to set up an equation using the Pythagorean theorem, try that out as a start...
     
  6. Oct 26, 2014 #5

    BvU

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    So the vector addition of ship's velocity wrt water + velocity of water = effective velocity of ship, right ?
    Write out the components in direction north and in direction east to get two equations with two unknowns (heading wrt water and effective speed). You don't need the latter, so eliminate it. I see the answer appearing without any difficulty ...

    Oh, and: welcome to PF :-)
     
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