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Homework Help: How do you find the amplitude?

  1. Jan 22, 2006 #1
    a block of mass 4kg is hung from a spring, causing it to stretch 8cm at equilibrium. the 4kg block is then replaced by a 5kg block. the accleration of gravity is 9.8m/s^2. how far will the 5kg block fall before its direction is reversed?

    i dont know if this helps but i found K for the first block


    i dont know anything else. HELP PLEASE:confused:
  2. jcsd
  3. Jan 22, 2006 #2
    8 cm is .08 m, not .8 m.
    Just find the distance to the from the old equilibrium point to the new one (the amplitude of oscillation) and multiply by two.
  4. Jan 22, 2006 #3

    how do you find the amplitude?
  5. Jan 23, 2006 #4
    Use the conservation of energy principal:
    In the beggining of the motion, the spring is unstretched, so it has no energy whatsoever, but the mass has potential energy m*g*x, where x is the distance it will fall before coming to a halt.
    In the end of the motion, the mass stops falling and is momentarily at rest, so it has neither potential nor kinetic energy. The spring, however, is now stretched by distance x from equilibrium, so the energy it has equals 0.5k*x^2. The energy at the beginning must be equal to the energy at the end, so you can write an equation and solve it to find x.
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