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Using the conservation of energy find speed

  1. Nov 10, 2012 #1
    1. The problem statement, all variables and given/known data
    A spring, having a force constant of 6.0x102 N/m, is held in a vertical position and compressed 0.30m. A 5.0 kg mass is then placed on top of the spring. THe mass is then releases. Neglecting air resistance and the mass of the spring

    3. The attempt at a solution

    Ek1 = Ek2
    1/2kx^2 = 1/2kx^2 + 1/2mv^2
    1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.2m)^2 + 1/2(5 kg)v^2
    27 J - 12 J = (2.5 kg)v^2
    2.45 m/s=V

    Im confused if we need to add any sort of gravitational potential energy to the ball?
     
  2. jcsd
  3. Nov 10, 2012 #2

    PhanthomJay

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    You need to consider both gravitational and spring potential energies and kinetic energy. Are you trying to determine the speed when the mass leaves the spring?
     
  4. Nov 10, 2012 #3
    yes, the velocity when it has moved up 0.20 m from the compressed position on the spring.
     
  5. Nov 10, 2012 #4

    PhanthomJay

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    When it moves up 0.2 m, the spring is now compressed by how much? Don't forget the gravitational PE.
     
  6. Nov 10, 2012 #5
    umm.... 0.1?
     
  7. Nov 10, 2012 #6

    PhanthomJay

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  8. Nov 10, 2012 #7
    so
    Ek1 = Ek2
    1/2kx^2 = 1/2kx^2 + 1/2mv^2
    1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.1m)^2 + 1/2(5 kg)v^2
    27 J - 3 J = (2.5 kg)v^2
    3.098 m/s=V
     
  9. Nov 10, 2012 #8

    PhanthomJay

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    What happened to the gravitational potential energy term you were inquiring about??
     
  10. Nov 10, 2012 #9
    umm what do you mean?
     
  11. Nov 10, 2012 #10
    Ek1 = Ek2
    1/2kx^2 = 1/2kx^2 + 1/2mv^2
    1/2(600N/m)(0.3m)^2 = 1/2(600N/m)(0.2m)^2 + 1/2(5 kg)v^2 + (5 kg)(9.8 m/s^2)(0.2 m)
    27 J - 3 J - 9.8 J= (2.5 kg)v^2
    2.38 m/s=V
     
  12. Nov 10, 2012 #11

    PhanthomJay

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    Looks good now!
     
  13. Nov 10, 2012 #12
    thanks :d
     
  14. Nov 12, 2012 #13
    Tell me who u are, u go to my school. DONT IGNORE ME
     
  15. Nov 12, 2012 #14
    ? who r u?
     
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