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Homework Help: Airplane (was: Moving Bullet)

  1. Nov 28, 2006 #1
    1. The problem statement, all variables and given/known data

    A Lufthansa A320 accelerates from rest to lift-off speed of 72.99 m/s in 25.00 s. Each of the plane's two jet engines provides a forward force (thrust) of 103.00 kN. What is the mass of the plane?

    How far does it travel down the runway before lift-off?

    2. Governing equations
    Vf=Vi+at
    EF=ma
    D=Vit+at^2

    3. The attempt at a solution
    Vf=72.99
    Vi=0
    a=?
    t=25.00s
    I used Vf=Vi+at to get a=2.9196
    then EF=ma to solve for mass and got 35.27880532 which i was told was wrong i went back and looked at the problem and thought that since the plane has two engines i would double the mass, but it was still wrong. and i tried D=Vit+at^2 for the second problem, but it didn't work either i don't understand what i'm doing wrong. any help would be appreciated
     
    Last edited: Nov 28, 2006
  2. jcsd
  3. Nov 28, 2006 #2

    Doc Al

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    Staff: Mentor

    So far, so good. (Don't forget units.)
    F = ma is the right approach. But what's the total F? There are two engines, each giving their own force. And be careful with units: 1 kN = 1 kilo-N = 1000 N.
     
  4. Nov 28, 2006 #3
    I think it should read like this:

    103kN x 2 = 206kN
    206kN = 206000 Newtons (N)

    F=ma

    206000 N = (m) (2.9196 m/s^2)

    m = 206000 N / 2.9196 m/s^2
     
  5. Nov 28, 2006 #4
    ok i got the mass of the plane to be 70557.61063kg, but i still don't know what i'm doing wrong to get the distance traveled down the runway.
     
  6. Nov 28, 2006 #5
    d = (1/2)(vi + vf)(t)
     
  7. Nov 28, 2006 #6
    i got it 912.375m thank you!!!!!!
     
  8. Nov 28, 2006 #7

    Doc Al

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    Staff: Mentor

    You had a mistake in your distance equation:
    That should be D = Vit + 0.5at^2.
     
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