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The confusing motion equation

  1. Oct 10, 2015 #1
    1. The problem statement, all variables and given/known data
    The picture of the question is given

    2. Relevant equations
    simple motion equations

    3. The attempt at a solution
    I am so confused so I cant solve the question
     

    Attached Files:

  2. jcsd
  3. Oct 10, 2015 #2
    Let t1 = 1.75 / 3.5 the time for the first person to reach the door
    t2 = (2 * 1.75 - .25) / 3.5 time for second to reach first person
    t3 = (3 * 1.75 - 2 * .25) / 3.5 time for third person to reach second person
    Can you arrange this into a geometric series?
     
  4. Oct 10, 2015 #3
    well I didnt see why theres a relationship between water level and the distance to person each other or simply L.
     
  5. Oct 10, 2015 #4
    ##t_n=n.1.75-(n-1).0.25)3.5##
     
  6. Oct 10, 2015 #5
    I found the b) its 9
     
  7. Oct 10, 2015 #6
    but stilll I didnt understand where that equation come from and a option
     
  8. Oct 11, 2015 #7

    haruspex

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    The 1.75 is the gap between people, so you don't need to subtract .25 from it.
    And I don't think you mean a geometric series.
    Water level?
    "Depth" here means in a horizontal direction. If there are three people pressed one against the other at the door then the depth is three people, or 3x.25m=0.75m.
     
  9. Oct 11, 2015 #8
    No, I don't think you need a series (arithmetic not geometric).
    If everyone starts moving at the same time then you should only need to calculate the
    time required for the n'th person to reach layer of people at the locked door.
    Incidentally, I subtracted the .25 to account for the increase in depth of people at the door.
    Since the third person would travel (3 * 1.75 - 2 * .25) because there was a .50 layer of people
    already at the door.
     
  10. Oct 11, 2015 #9

    haruspex

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    How far from the door is the third person at time 0?
     
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