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Homework Help: Motion - V.Confused!

  1. Jan 4, 2010 #1
    1. The problem statement, all variables and given/known data
    An aircraft of mass 5.8 x 10^4 kg stands at the end of the runway ready for take-off. The engines provide a constant total thrust of 2.1 x 10^5N

    i)Calculate the initial acceleration in ms-2 of the aircraft as it starts to move

    ii) Calculate the length in m of runway needed for the aircraft to reach its take off speed of 85 ms-1 , assuming acceleration is constant

    2. Relevant equations
    Motion Equations

    v = u + at
    s = (u + v)/2 * t
    s = ut + 1/2*at^2


    3. The attempt at a solution

    The answer is 3.6ms to part i) but i cant seem to get anywhere near that.
    I really dont know where to start.
    Do i square both numbers and square root it? I just need a guide to lead me what info i have and what info i need.

    thank you
     
  2. jcsd
  3. Jan 4, 2010 #2

    mgb_phys

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    Part (i) Have you tried F=ma ?
     
  4. Jan 4, 2010 #3

    rock.freak667

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    Newton's 2nd law, F=ma
     
  5. Jan 4, 2010 #4
    for i) use f=ma you know the force from thrust and the mass of the object so rearrange to give
    a=f/m
    that will give you your acceleration then substitute that into you motion equations
    you have a, u and v. you want to find t
    v=u+at
    rearrange to give t
     
  6. Jan 4, 2010 #5
    wow..
    i just figured it out myself just then.

    i look very stupid now :\

    Ok i got part i) done but what about part ii) ?
     
  7. Jan 4, 2010 #6
    read above
     
  8. Jan 4, 2010 #7
    Ok so i got these

    a: 3.6ms
    u: 85ms
    v: 0ms?
    t: ?

    Is that the right layout?
    But why would you want to find 't'? when your asked to find the 'length' ?
    I assume you get the value for 't' and substitute into s=ut+0.5at^2?

    Ps. How do you distinguish something as an initial velocity(u) and final velocity(v)?
    Eg. The take off speed of 85ms in the question.
     
    Last edited: Jan 4, 2010
  9. Jan 4, 2010 #8

    mgb_phys

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    There is another of the motion equations with V,U,a and distance.
     
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