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Spring Problem (Hookes law)

  1. Jul 24, 2017 #1
    1. The problem statement, all variables and given/known data
    A block of mass M is hung on a vertical spring.
    We have to find spring constant k, x (displacement from mean position) given.

    2. Relevant equations
    1/2kx^2 = mgx
    mg=kx

    3. The attempt at a solution
    When i conserve energy,
    1/2kx^2=mgx
    ⇒ k= 2mg/x

    But when i use mg=kx,
    I get k=mg/x

    But mg/x≠2mg/x,
    Help please :)
     
  2. jcsd
  3. Jul 24, 2017 #2
    You don't need to compute energy here.
    Just use Hooke Law directly.

    What force is M applying to the spring?
    How far is this causing the spring to stretch?

    The assumption here is that the system is stable at x.
     
    Last edited: Jul 24, 2017
  4. Jul 24, 2017 #3
    after that it is given some velocity v from down, i need to calculate how high will it go.
    i used 1/2kx2=1/2mv2 is it correct?
     
  5. Jul 24, 2017 #4

    haruspex

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    Is that based on some reasoning or just a wild guess?

    If it rises above the equilibrium point by a maximum of h, what are the changes in
    1. KE
    2. GPE
    3. Elastic PE
    ?
     
  6. Jul 25, 2017 #5
    Consider the situation of the mass moving horizontally along a frictionless surface.
    What is the difference in the two situations?
     
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