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Hooke's Law Elevator Spring Question

  1. Nov 21, 2015 #1
    1. The problem statement, all variables and given/known data
    Elevator initially at rest.
    Equilibrium length L0=40.0cm
    60-kg person stands on spring
    L1= 32.0cm

    The elevator than speeds upwards at 2.50 m/s2
    What is the new length (L2)

    2. Relevant equations
    Fnet=ma
    Fsp=-kdeltaX
    FG=mg
    3. The attempt at a solution
    Taking down as positive y hat direction
    Before:
    Fnet=Fsp+FG
    0=-k(0.32-0.40)+(60.0)(9.81)
    K=-7358
    After:
    Fnet=Fsp+FG
    ma=-kdeltaX+mg
    m(-2.5)=-(-7358)(L2-0.40)+60(9.81)
    L2=[{60(-2.5)-60(9.81)}/-(-7358)]+0.40
    =0.30m from equilibrium

    I am confused since i don't know how this makes sense according to the coordinate system. (I know the answer is right as my prof posted the answer, not the work, just answer)
    At rest change in x = -0.08
    When accelerating change in x= -0.10
    But since my coordinate system is set up so Y-hat is positive is the downward direction doesn't this mean my spring is being displaces upward?
    This doesn't make sense to me , can someone please explain very clearly?
     
  2. jcsd
  3. Nov 21, 2015 #2
    Take a look at your signs again. Are they consistent with the defined direction?
     
  4. Nov 21, 2015 #3
    I kinda see what you are getting at
     
  5. Nov 21, 2015 #4

    SteamKing

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    Staff Emeritus
    Science Advisor
    Homework Helper

    If you're standing in an elevator when it starts to go up, do you feel heavier or lighter when the car starts to move? Is the spring therefore going to get longer or shorter as a result?

    You seem to have sprinkled a generous helping of negative signs throughout your calculations without systematically considering coordinate systems.
     
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