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Vibrations and waves

  1. Aug 28, 2009 #1
    Vibrations and waves
    1) A 5 kg mass, m, rests on a frictionless, horizontal, wooden table top, and is attached to one end of a spring anchored at its other end . If the spring has a spring constant=13 N/cm and the mass is pulled back (in the positive direction ) 20 cm and released, to the nearest tenth of a joule, what is its total energy?
    2) if the mass is 15 kg and the spring constant is 20 N/cm, to the nearest tenth of a m/s, what is its speed at x= 5 cm?

    I have solved the 1st one :


    E = PE + KE
    E = 1/2kA^2 + 1/2mv^2 At KE v=0 because the mass stops momentarily as it changes directions. Then :

    E = PE
    E = 1/2kA^2
    E = 1/2(13*100)*(0.2)^2
    E = 26J

    I have problem with the second problem. please help to solve this. I have 2 hours that im tryin to solve it . I have only the answer that is 2.2m/s.

    Please help.......................

  2. jcsd
  3. Aug 28, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Find the total energy, just like you did for part one. Then use:
    Solve for v, when x = 5 cm.

    That equation should be: E = 1/2kx^2 + 1/2mv^2. Use x, not A (which usually stands for amplitude).
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