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Homework Help: Kinematics Acceleration (27)

  1. Jul 27, 2016 #1
    1. The problem statement, all variables and given/known data
    A train that runs 140 km / h requires about 700m to stop. Calculate the average acceleration of the train while braking.


    2. Relevant equations
    v: x/t



    3. The attempt at a solution
    v= x/t
    so t= x/v= 700m/ (38.89m/s)=18 s
    Now to find acceleration a= v/t
    a= 38,89m/s / 18s =2,160 m/s^2
    They're asking the average so 2,160 m/s2 is our initial acceleration
    therefore
    0-2,160 m/s^2 / (2) = -1,08024
    Is it the correct formula to use ? or is it wrong ???
     
  2. jcsd
  3. Jul 27, 2016 #2
    The equation you used for the velocity is correct only if the acceleration is zero (i.e., only if the velocity is constant).
     
  4. Jul 27, 2016 #3
    Oh I see. I think i've found out which kinematic equation to use .

    I tried this one
    initial velocity = 140 km/h (38,89 m/s)
    Final velocity = 0 m/s because its going to stop
    (vx)^2=(u)^2 + 2x a x700 m
    (38,89)^2=1400 x a
    a= -1512,34/ 1400
    a= -1,0802
     
  5. Jul 27, 2016 #4

    PeroK

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    Gold Member

    That's correct. Can you think of a way to confirm that answer is right?
     
  6. Aug 3, 2016 #5
    My workbook says so
     
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