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Weight in N with change in acceleration

  1. Jan 30, 2006 #1
    2kg weight is suspended on a tension scale that measures in Newtons.
    The elevator is moving at constant velocity up of 10m/s. Its velocity is then uniformly reduced to zero in 4 seconds. So that it comes to rest in 4 seconds. What does the scale read during those 4 seconds.

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    Using frame of reference with positive down: (That way gravity's acceleration is positive.)
    Using Kinematics:
    Vf=Vi*t+1/2a*t^2
    Therefore:
    a=(-2Vi)/t=-5m/s^2
    Applying F=ma:
    Ftension+Fgravity=ma
    -Ft+mg=ma
    Ft=mg-ma=2*9.8-2*(-5)=29.6N

    My books says: 14.6N

    What'd I do wrong?
     
  2. jcsd
  3. Jan 31, 2006 #2

    Meir Achuz

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    Your first equation is wrong.
    It should be Vf=Vi+at.
     
  4. Jan 31, 2006 #3
    Now i feel kinda dumb. I knew that I swear....that first equation is for displacement...doh!
     
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