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Relative motion or newton first law?

  1. Dec 10, 2015 #1
    1. The problem statement, all variables and given/known data

    An observer on the ground sees a person on a train just leaving the station in the east direction. explain how the observer on the ground could see the person moving to the west while the train is moving east

    2. Relevant equations


    3. The attempt at a solution

    At first I thought this was Newton First law. Reason is because the guy the person is at rest but when the train starts moving east, the person is jerked backward and therefore he moved west. However the question doesnt say the train was suddenly acceleration forward so i dont believe this is the right answer.

    I then thought about how this can also be seen as relative motion: the person is already moving west. The person must also be walking at a speed greater than the speed of the train to make it look like the person is moving west. Then when the train starts to accelerate and once the trains speed is greater that the person's, the person will no longer be seen moving west

    I am torn on which answer it is
     
  2. jcsd
  3. Dec 10, 2015 #2

    haruspex

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    In the first suggestion, would the jerk result in the passenger moving West relative to the train faster than the train is moving to the East?
     
  4. Dec 10, 2015 #3
    I believe so. Your wording is a bit confusing :s
     
  5. Dec 10, 2015 #4

    haruspex

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    Let me phrase it with an analogy.
    A marble is at rest on a horizontal flat plate. You tug the plate swiftly to the right. Do you believe that this could result in the marble's moving, even briefly, leftwards relative to the ground?
     
  6. Dec 10, 2015 #5
    Definetely
     
  7. Dec 10, 2015 #6

    haruspex

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    If that's your intuition then I can only suggest you work through some equations to see what will happen.
     
  8. Dec 10, 2015 #7
    How about the second option that I've written? In both cases the person can be seen moving to the west even when the train moves to the east. Is one answer more right than the other?
     
  9. Dec 10, 2015 #8

    haruspex

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    I'm quite happy with your second answer. The first is provably wrong.
     
  10. Dec 10, 2015 #9
    thank you
     
  11. Dec 10, 2015 #10

    haruspex

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    OK, but your answer in post #5 bothers me. You need to educate your intuition on this.
    The only force acting horizontally on the marble is to the right. How can that make it move left?
     
  12. Dec 10, 2015 #11
    well if i think about a car on the highway, when we accelerate even more, our body moves slams backward in the seat before our bodies adapt to the change in velocity. But now that I think about it, I'm not quite sure where the marble would move left then right
     
  13. Dec 10, 2015 #12

    haruspex

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    Yes, but that's motion relative to the car, not relative to the ground.
     
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