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Homework Help: Need help with a Slope problem

  1. Sep 23, 2007 #1
    Need urgent help with a Slope problem

    1. The problem statement, all variables and given/known data
    A plane is traveling at a constant velocity at 30 degrees above the horizontal. Weight (W)=86 500 N; thrust (T)=103 000 N. Find the lift force and the resistance force (L= normal force, R=friction) without shifting the cartesian coordinate system


    2. Relevant equations
    sum of the forces in the x=ma, sum of the forces in the y=ma. a=0.


    3. The attempt at a solution
    I have set up the problem with x dir: T cos(30) - L sin(30) - Rcos(30)=ma and y dir: W - T sin(30) + L cos(30) - R sin(30)=ma and ma=0. R in the y dir is negative. Is R in the y dir equivalent to mg? I'm stuck with too many variables in my equation. Any help would be greatly appreciated.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 23, 2007 #2

    Astronuc

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    Science Advisor

    W = mg and it acts directly downward. Lift acts vertically. Resistance (Drag) acts in opposition to motion, or against the thrust.

    See - http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html

    http://www.grc.nasa.gov/WWW/K-12/airplane/presar.html
    By convention take T and L to be positive, and R and W (= mg) to be negative.

    See also - http://www.grc.nasa.gov/WWW/K-12/airplane/cruise.html
    which gives the condition for straight (horizontal) flight.
     
  4. Sep 23, 2007 #3
    I understand the concepts as you explained them. My problem is that I connot tilt the x,y axis to solve the problem with information I have. Therefore, I am trying to use trig functions to solve for T and R, which are not directly on the x and y axis. The only compnent that is on the x and y axis is W. Since R in the y direction is negative, can I use mg to solve R (in the y direction) sin (30)?
     
  5. Sep 23, 2007 #4
    Anyone?
     
  6. Sep 23, 2007 #5

    Astronuc

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    Staff Emeritus
    Science Advisor

    What does the text say about lift (L). According to NASA's site, L is vertical and balance against weight. The thrust (T) and drag or resistance (R) are oriented along the axis of the aircraft.

    So that would imply L = W and T = R for cruising or constant velocity.
     
  7. Sep 23, 2007 #6
    Thank you!! That is exactly the information I needed!! The problem worked out great.
     
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