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Frames of Reference on Top of a Moving Train

  1. Dec 16, 2009 #1
    When an object is thrown by someone standing on top of a moving train, which variations of "frames of reference" would apply? (i.e. inertial, non-inertial, etc.) How would this principle work? (The object would go the same relative distance as it would if thrown from a person standing on motionless ground, disregarding air resistance of course)
  2. jcsd
  3. Dec 16, 2009 #2
    You'll have to clarify the question. If the train is accelerating relative to the Earth, it is a non-inertial frame.
  4. Dec 16, 2009 #3
    The train isn't accelerating; its velocity is constant. More focus is placed on the action of the object being thrown forward in respects to the moving train than the motion of the train.
  5. Dec 16, 2009 #4


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    Staff: Mentor

    It is inertial motion and the principle of relativity applies: you can play "catch" in the cabin of a moving train exactly the same if it is moving or not.
  6. Dec 16, 2009 #5


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    Staff: Mentor

    Or on top of the train, provided of course that you're not going fast enough to make the wind a factor.
  7. Dec 17, 2009 #6


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    I never thought of this before, but this particular comment snagged my attention. To some extent, the train is, in fact, accelerating. It is following the curvature of the Earth, which means that its path is actually circular rather than linear. I might be misunderstanding some official terminology, but that constant 'downward' change of vector implies to me an acceleration.
  8. Dec 18, 2009 #7
    Assume a person on the platform uses energy E to throw a ball in the direction the train is heading and the ball lands on the ground 100 feet in front of him.
    Suppose again that a person standing on the train throws a ball in the same direction, using the same amount of energy to do so. Relative to the person on the train, the ball falls to the ground far closer than 100 feet.
    In light of this, how can the object "go the same relative distance..."?
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