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Elastic Potential- Acceleration with springs

  1. Nov 6, 2013 #1
    1. The problem statement, all variables and given/known data
    A 1.6kg mass is suspended from a spring inside a stationary elevator. The spring stretches 12cm. The elevator now accelerates upwards, and the spring extends a further 12cm. What is the elevator's acceleration?


    2. Relevant equations
    Fy= may
    Ue= 1/2 (kx2)

    3. The attempt at a solution
    I'm just not sure on the best way to start this question.
     
  2. jcsd
  3. Nov 7, 2013 #2
    I would start with the first equation you provided. [itex]F_y = ma_y[/itex]. What's the acceleration ([itex]a_y[/itex]) in this problem? Where you can find a force on the system?
     
  4. Nov 7, 2013 #3
    Fy=may
    kx-mg=may
    ay=(kx-mg)/m

    but k is not given....
     
  5. Nov 7, 2013 #4
    You need to find k first. What's the spring doing BEFORE the elevator starts moving?
     
  6. Nov 7, 2013 #5
    Fy=may
    kx-mg=0
    k=(mg)/x
    k=1306.666 N/m

    Fy=may
    kx-mg=may
    ay=(kx-mg)/m
    ay=9.79999 m/s2
     
  7. Nov 7, 2013 #6
    I got a different value for k, could you double check it? My answer was a magnitude different.
     
  8. Nov 7, 2013 #7
    oops, so k=130.666 N/m
     
  9. Nov 7, 2013 #8
    Right, and you also know the distance so now would be a good time to find the potential.
     
  10. Nov 7, 2013 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Looks to me like you're done. No need for the potential in this problem.
     
  11. Nov 7, 2013 #10
    Ah, yep. I was hasty.
     
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