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Train inertial referance frame

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data

    You are in a train traveling on a horizontal track and notice that a iece of luggage starts to slide toward the front of the train. from this observation you can conclude that this train is not an inertial reference frame because it is slowing down.

    2. Relevant equations

    true

    3. The attempt at a solution its true but why??
     
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  3. Oct 10, 2011 #2

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    Uhh... what's the definition of an inertial reference frame?
     
  4. Oct 10, 2011 #3
    a FRAME REFERANCE THAT DESCRIBES TIME AND SPACE
     
  5. Oct 10, 2011 #4
    it has no acceleration
    and for the crate to move there must be deacceleration
     
  6. Oct 10, 2011 #5

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    Hmm, let's take a look at wikipedia.
    It says (skipping a couple of paragraphs):

    "In an inertial frame, Newton's first law (the law of inertia) is satisfied: Any free motion has a constant magnitude and direction"

    Does the piece of luggage have a motion with constant magnitude and direction?


    Turning this around: if the train deaccelerates, the crate would move.
    What if the train accelerates?
     
  7. Oct 10, 2011 #6
    if the train accelerates it would stay still
     
  8. Oct 10, 2011 #7

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    Have you ever been standing in a train or bus that accelerates?
    If you have, did you have trouble to keep standing?

    Or in a car, can you feel it if the car accelerates?
     
  9. Oct 10, 2011 #8
    um it was a while algo, but is it like when it takes off it acclerates so you kinda jerk forward?
     
  10. Oct 10, 2011 #9

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    Well, if a train accelerates, you are jerked forward.
    And if you don't compensate you'll be flying to the back of the train.

    In a car you are pressed against the back of the seat.
    And when a car is braked, you'd fly forward except that the safety belts keep you in place.
     
  11. Oct 10, 2011 #10
    okay so for the luggage to slide forward you must be accelerating
     
  12. Oct 10, 2011 #11

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    Well... a piece of luggage would not try to compensate...
     
  13. Oct 10, 2011 #12
    so it would just slide back then
     
  14. Oct 10, 2011 #13

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    Yes - if the train were accelerating, the piece of luggage would slide back.

    What was the question again?
     
  15. Oct 10, 2011 #14
    why can you conclude it is not an initial referance frame?
     
  16. Oct 10, 2011 #15

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    Not exactly...

    The question starts by stating that the piece of luggage starts moving forward.
    ... and it ends with the question whether the train is slowing down.

    Does it?
     
  17. Oct 10, 2011 #16
    yes.because its the opposite of it moving forwards
     
  18. Oct 10, 2011 #17

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    Right! :smile:

    As for the inertial reference frame question:

    Does the piece of luggage have a motion with constant magnitude and direction?
     
  19. Oct 10, 2011 #18
  20. Oct 10, 2011 #19

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    Well, it's rather important that the piece of luggage "starts" moving.
    That means that its speed (or motion) is *not* constant...
     
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