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Need help on harmonic motion problems

  1. Apr 21, 2005 #1
    please help me with these two problems:

    1. A meter stick is hung at its center from a thin wire It is twisted and oscillates with a period of 6.06 s. The meter stick is sawed off to a length of 72.4 cm. This piece is again balanced at its center and set in oscillation. With what period does it oscillate?

    2. A 53.7 kg person jumps from a window to a fire net 20.4 m below, which stretches the net 1.19 m. Assume that the net behaves like a simple spring, and calculate how much it would stretch if the same person were lying in it.

    for number one, I tried to set up a ratio T1/T2 = sqrt(I1/I2) where T1 is the period given and I am trying to find T2, but I=(1/12)ml^2 but I can't find the new mass for I2.

    for number 2 I thought I could set up a energy equation where
    mgh = (1/2)Kx^2 where x = 1.19 and h = 20.4, then I solved for K. once I got K I used K = mg/l to solve for l but I get the wrong answer. please help!!
  2. jcsd
  3. Apr 21, 2005 #2

    Doc Al

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    Staff: Mentor

    You don't need the mass, just the ratio between the masses. If a meterstick (100 cm) has a mass of m, what must be the mass of the 72.4 cm piece? (Assume the meterstick has uniform density.)

    Calculate the gravitational PE from the lowest point of the motion.
  4. Apr 21, 2005 #3
    thanks I got the first one, but I still don't understand the second one. what do you mean the lowest point of motion?
  5. Apr 21, 2005 #4
  6. Apr 21, 2005 #5
    thanks so much for your help.
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