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Reference frame

  1. Oct 30, 2009 #1
    1. The problem statement, all variables and given/known data

    An airplane, whose air speed is 580 km/h, is supposed to fly in a straight path 38 degrees N of E. But a steady 72 km/h wind is blowing from the North. In what direction should the plane head?

    Relevant Equations
    Pythagorean theorem
    Law of sines

    Attempt at a solution
    The first thing I did was to draw myself a diagram so that I could see what is happening in this problem. I tried to attach a diagram that I made myself, but that did not work out at all.
    What I did was to label each one Velocity of the plane with respect to the ground was pointing in the N of E direction and was approximated to 38 degrees, Velocity of the air with respect to the ground was place so that it was head to tail of the previous vector and pointed in the southward direction and the last one was labeled velocity of the plane with respect to the air and it was placed facing in the Eastern direction located directly onto the x-axis. Once I resolve it into components I am getting Y: [tex]V_{pg}[/tex]*sin([tex]\theta[/tex])-[tex]V_{ag}[/tex]*sin([tex]\theta[/tex]), for the x component I found [tex]V_{pg}[/tex]*cos([tex]\theta[/tex]).I plugged my numbers into the equations and found my y components to have a value of 312.76 km/h and my x components to have a value of 457.05 km/h. I took the arcsin(y/x) and found the direction to be 43.2 degrees this troubles me because the solution is supposed to be 43.6 degrees. I need to know how to fix this error so that I don't make mistakes like this in the future I would appreciate any assistance that can be offered. Thank you for your time.
     
  2. jcsd
  3. Oct 30, 2009 #2
    Was this a textbook question and was it the answer found in the back of the book?
    And did you use exact values or rounded values... always use exact values use the calculators memory or store function.
     
  4. Oct 31, 2009 #3

    Andrew Mason

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    You have to show us how you worked it out. It strikes me that this is a rather difficult vector problem. Unless you show us exactly what you did, it is difficult to find out where you went wrong. What did you find to be the magnitude of the plane to ground velocity? How did you determine that?

    AM
     
  5. Oct 31, 2009 #4
    Re:frame of reference continuation

    Alright, sorry I was finally able to get an image onto this site, and built up the courage to let you see how wrong I am. I am certain that I am doing something wrong and I have looked at this problem so many times that I thought my eyes would cross. I have to know what is wrong with my logic. Please help anyone, I do not want a solution there is one in the back of my text, I want to understand. I also forgot to include the information Adrew Mason asked about. I have found on few pieces of paper that have now become garbage Vpa, I changed my diagram, used conversions then not using conversions and at some point I know I began to make up some rules of my own out of desperation. I am lost!
     

    Attached Files:

    Last edited: Oct 31, 2009
  6. Oct 31, 2009 #5
    Thank you for the last part of your comment it got me really thinking about it again and now I have something more concrete to try, fingers crossed.
     
  7. Oct 31, 2009 #6

    Andrew Mason

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    Re: frame of reference continuation

    What you have to do is calculate the length of the plane-ground vector. Call it L. Call the wind velocity vector (length of the air-ground velocity vector) W and the plane-air velocity vector P.

    Write out the expression for L in terms of W and P and the known angle. You will get a quadratic equation in terms of L. Find the solution of that equation and then you can determine the angle of P. The correct answer is 43.6 degrees north of east.

    AM
     
  8. Oct 31, 2009 #7
    Re: frame of reference continuation

    Hey, I was trying this as well, I was onto something but I gave up.. could you show your work please?

    thanks
     
  9. Oct 31, 2009 #8

    Andrew Mason

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    Re: frame of reference continuation

    You have a vector of indeterminate length going in a direction of 38 degrees north of east (plane-ground velocity - call it L). You have a vector of length 72 (km/hr) from the tail of the previous vector heading due south (W). The difference is the plane-air velocity vector (P)

    W + P = L

    Work out the length of P in terms of W and L:

    P^2 = (W + Lsin38))^2 + (Lcos38)^2

    Work that out to find L. You have to solve the quadratic equation (Hint: what is cos^2 + sin^2)?

    AM
     
  10. Oct 31, 2009 #9
    There is an attachement in one of the previous parts of this post but I have figured out how to solve it now, and an putting the finishing touches on it, so that it is accurate in everyway. Also I was really confused when I started the problem, take another look at your diagram. My original diagram was causing me to solve the problem wrong. I don't know about just giving out an answer like that, they made me work for mine, now you work for yours. Nothing personal it will just make you better. If you still can't get it in a while, then I will attach my new diagram.
    Sorry that message is for emyt, thank you guys very much I appreciate being able to come to a forum where people don't just blurt out answers. Thanks for all the insight, have a great week-end.
     
    Last edited: Oct 31, 2009
  11. Oct 31, 2009 #10
    Re: frame of reference continuation


    Ahhh I see, I have a diagram like this drawn, I just didn't make the connection.. thanks
     
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