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Homework Help: Who I can find the displacement in this Q ?

  1. May 29, 2010 #1
    Hi all



    who I can find the displacement in this Q ?



    A jetliner lands with a speed of 90 m/s on runway . It runs with constant speed for 2 s , before the brakes are applied . The brakes produce a deceleration of 20 m/s2 . What is its displacement on the runway .



    I try to solve



    V0 = 90 m/s

    t1 = 2s

    x = V0 X t1 = 180 m

    a = -20 m/s2



    then I don't know who I complte it ?
     
  2. jcsd
  3. May 29, 2010 #2

    tiny-tim

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    Hi r-soy! :smile:

    (btw, it's how (rhyming with now and Mao) not who (rhyming with new and you) :wink:)
    This is how

    The total displacement is the sum of the two displacements.

    Your first displacement is ok.

    For the second displacement, use one of the standard constant acceleration equations, with distance = s, acceleration = a = -20, u = 90, and v = 0. :wink:
     
  4. May 30, 2010 #3
    see

    Here I must find two displacement .
    firist one I got it
    now the secound now

    I have Dx1 = 180 and a = -20, u = 90, and

    by equat5ion constant I use v^2 = v0^2 = 2aDx
    fainal velocity is 0 beacuse it will stop after using brake
    now :
    0 = 90^2 + 2(-20) Dx2
    Dx2 = 202.5

    Now collect two dispkacement : will get : 362.5 m
     
  5. May 30, 2010 #4

    tiny-tim

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    (try using the X2 tag just above the Reply box :wink:)

    Yes, that's fine :smile:

    (except 180 + 202.5 = 382.5 :wink:)
     
  6. May 30, 2010 #5
    Hmm not sure what you guys are talking about..but il get it later.
     
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