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How long does it take for the train to stop moving?

  1. Jun 27, 2007 #1
    1. The problem statement, all variables and given/known data
    a train is traveling down a straight track at 20m/s when the engineer applies the brakes resulting in an acceleration of -1.0m/s^2 as long as the train is in motion. how far does the train move durin g a 40 second time interval starting at the instant the brakes are applied?


    2. Relevant equations



    3. The attempt at a solution
    I'm new to physics and have no idea what to do. i keep getting zero when i plug it into the formula. also...i know that the train eventually stops before 40 seconds and doesn't travel anywhere...but how do i incorporate that into the equation?
     
  2. jcsd
  3. Jun 27, 2007 #2

    Doc Al

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    Staff: Mentor

    How long does it take for the train to stop moving?
     
  4. Jun 27, 2007 #3
    i think it takes 20 seconds...but i'm not so sure.
     
  5. Jun 27, 2007 #4

    Doc Al

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    That's correct. If you're not sure, review the definition of acceleration.
     
  6. Jun 27, 2007 #5
    okay...so if it takes 20 seconds for the train to stop moving...doesn't that mean it travels for a total of 210 meters?
     
  7. Jun 27, 2007 #6

    Doc Al

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    How did you deduce that? (Always show your work.)
     
  8. Jun 27, 2007 #7
    i'm using common sense for the problem but i don't know what equation to use to get the answer of 200 meters. i just did 20+19+18+...+1+0=210 meters.
     
  9. Jun 27, 2007 #8

    Doc Al

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    Commonsense is OK if you can justify it. What do those numbers mean? 20 = ?; 19 = ?...

    If you are thinking that the train travels 20 m in the first second, etc., that's not quite correct. Hint: What's the average speed during the first second?

    I suspect you should be learning some kinematic relationships.
     
  10. Jun 27, 2007 #9
    thanks so much...i figured it out...and finally understand what i'm doing with all my other hw problems!
     
  11. Jun 27, 2007 #10
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