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Homework Help: Very Simple Airplane problem

  1. Nov 20, 2011 #1
    1. The problem statement, all variables and given/known data

    A plane must reach a velocity of 2.4 km/h to lift off. If it has 45s of runway time, what must its acceleration be?

    vf= 2.4km/h or 0.666666666666 m/s

    vi=0.0 m/s


    a= ?
    2. Relevant equations

    vf= vi+ aΔt

    3. The attempt at a solution

    vf- vi
    ------- = a

    ---------------- = a
    45 s

    a= 0.014814814 m/s^2
    a= 1.5 * 10^2 m/s^2

    i did all of this but i am getting the wrong answer. Can someone tell me what i am doing wrong?
  2. jcsd
  3. Nov 20, 2011 #2
    a = 1.5 * 10^ (-2) m/s^2

    negative 2 not positive 2
    otherwise it seems fine
  4. Nov 20, 2011 #3
    the answer is 1.5 * 10 ^ 2 m/s^2 [E]. but they don't state which direction is negative or positive. so i am still right?
  5. Nov 20, 2011 #4
    Seems like two orders of magnitude error here. And that lift off speed is a very leisure walk speed. Could it be that the question reads:
    A plane must reach a velocity of 240 km/h to lift off.
  6. Nov 20, 2011 #5
    as for direction, if it didn't state which way the airplane was flying, then it doesn't matter
    I would say it is a bad question

    and for 10^-2

    the answer should be 10^-2 not 10^2
    because the answer was 0.015

    think about it, 10^-2 =1/(10^2) = 1/100 =0.01
    so the answer should be 1.5 x 0.01 =0.015
  7. Nov 20, 2011 #6
    I got it directly from the worksheet i have and it says it must reach a velocity of 2.4 km/h. I know it is not realistic in a real world, but considering that the teacher just made up numbers so we can practice.
  8. Nov 20, 2011 #7
    yes maybe that is the problem
    the airplane cannot fly when its speed is slower than a hamster on treadmill
  9. Nov 20, 2011 #8

    ye i agree with you
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