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Homework Help: Elastic potential energy of a spring

  1. Feb 25, 2010 #1
    Hello,
    I'd relly appreciate some guidance on this one:

    1. The length of a spring increases by 7.0 cm from its relaxed length when a mass of 1.7 kg is hanging in equilibrium from the spring. Spring constant 2.38 N/cm.
    How much elastic potential energy is stored in the spring?
    A different mass is suspended and the spring length increases by 12.9 cm from its relaxed length to its new equilibrium position. What is the second mass?



    2. Relevant equations
    PE=.5kx^2


    3. The attempt at a solution
    .5(2.38)(7)^2=58.31
     
  2. jcsd
  3. Feb 25, 2010 #2


    You must change the units for your PE calculation. Your current answer is [itex]\rm N\cdot cm[/itex], but you should write it in terms of [itex]\rm J = N\cdot m[/itex].

    To figure out the unknown mass, draw a free body diagram, and then apply Newton's 2nd postulate for static equilibrium.
     
  4. Feb 25, 2010 #3
    What forces are there acting on the mass? Obviously there is the spring force (up), but is there another force on the mass thats keeping the spring stretched? As dr_k said, drawing an FBD will help you sort this weighty problem out.
     
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