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Homework Help: Mass and spring

  1. Jan 10, 2010 #1
    1. The problem statement, all variables and given/known data
    A 0.25 kg mass is attached to a spring with spring constant 5.4 N/m and let fall. To the nearest hundredth of a meter what is the point where it 'stops'?

    diagram here:
    http://wps.prenhall.com/wps/media/objects/1088/1114633/ch11/grav.gif


    2. Relevant equations
    y=Asin(k/m)1/2t


    3. The attempt at a solution
    I don't know how to find t, or I am using the wrong equation.
     
  2. jcsd
  3. Jan 10, 2010 #2
    What is going on(Force wise) when the mass reaches its lowest point?
     
  4. Jan 10, 2010 #3
    I tried F=-kx using mg for force. mg is (.25)(9.8) which equals 2.45. Dividing by 5.4 gave me .45 which is wrong. The correct answer is .91
     
  5. Jan 10, 2010 #4
    It is a giancoli problem so I can reset the variables. I've tried it multiples times and the F=-kx is always half of the correct answer. Where does it get multiplied by 2.
     
  6. Jan 10, 2010 #5

    PhanthomJay

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    Your equation assumes the weight is slowly lowered to its equilibrium position. In this case, however, it is dropped suddenly. Use conservation of energy to show that its total stretch is twice the stretch in its at rest equilibrium position.
     
  7. Jan 10, 2010 #6
    i have .5mv2=.5kx2 but i don't know how to find velocity.
     
  8. Jan 10, 2010 #7

    PhanthomJay

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    The object starts from rest with no speed, and when it reaches the bottom, it momentarily stops and also has no speed. But there is gravitational potential energy at the top, and spring potential energy at the bottom.
     
  9. Jan 10, 2010 #8
    Thank You, that gave me the right answer.
     
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