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Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, when,

  1. Aug 28, 2008 #1
    1. The problem statement, all variables and given/known data
    Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, when the engineer spots a cow on the track. The train brakes to a halt in 2.3 min, stopping in front of the cow.

    a) What is the magnitude of the train's (constant) acceleration?
    _______m/s^2 <-----(0.22m/s^2) Correct

    b) What is the direction of the acceleration?
    - eastward
    - westward <-----Correct
    - southward
    - northward

    c) How far was the train from the cow when the engineer first applied the brakes?
    ______km

    3. The attempt at a solution
    I have a) and b) correct but I can't figure out c).

    Thanks.
     
  2. jcsd
  3. Aug 28, 2008 #2
    Re: Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, w

    reverse it. imagine the train is accelerating at 0.22 m/s/s
     
  4. Aug 28, 2008 #3

    LowlyPion

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    Re: Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, w

    First what equation do you know that relates distance acceleration and time?

    (Btw, I think the 20th Century Limited stopped running in 1967. Today it's the Lake Shore Limited that runs the route.)
     
  5. Aug 28, 2008 #4
    Re: Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, w

    I know this equation but I get the wrong answer: d=(vf^2-vi^2)/2a
     
  6. Aug 29, 2008 #5

    LowlyPion

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    Re: Amtrak's 20th-Century Limited is in route from Chicago to New York at 112 km/h, w

    I would have simply used the x = 1/2 a t2 relationship.

    x = 1/2 * .22544 * (138 sec)2

    The other equation works as well

    x = (31.111)2/(2*(.22544))
     
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