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Homework Help: Out of ideas, and frustrated

  1. Apr 20, 2008 #1
    Out of ideas, and frustrated!!!!!!!!

    1. The problem statement, all variables and given/known data
    A ball dropped from a height of 4.00 m makes an elastic collision with the grond. Assume that no mechanical energy is lost due to air resistance. Determine the period of the motion

    2. Relevant equations

    I know the period (T) = the time required for the object to make one cycle.

    3. The attempt at a solution

    I get: 4m/9.8 m/s^2=.408 s^2 s=.639 (square root of .408) .639 x 2= 1.278 s (time for it to go down 4 m and up 4 m)
    My book gives 1.81 s as the answer. I can get that if I take the square root of 8/9.8 (8 being the total distance traveled in one period) and multiply it by 2. But why? That would be the time required for 2 periods, wouldn't it? I'm banging my head againstthe wall on this one!!!
  2. jcsd
  3. Apr 20, 2008 #2
    you can calculate the time for it to reach the ground from the kinematic eq:

    y = 1/2 at^2. The time for this will be the way down, or 1/2 the cycle.
  4. Apr 20, 2008 #3
    That answers it! I knew I was missing something stupid!
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