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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


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


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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
    that will give you your acceleration then substitute that into you motion equations
    you have a, u and v. you want to find t
    rearrange to give t
  6. Jan 4, 2010 #5
    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


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