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Homework Help: Mechanical energy in a pendulum

  1. Jan 30, 2010 #1
    A pendulum was set up and measurements were made to enable the mechanical energy to be calculated at the start position S and the lowest point of the pendulums swing L.

    The mass of the pendulum bob was determined on an electronic scale and its diamter was measured using calipers. The initial height was measured with a meter stick. At the lowest point of its swing, the pendulum bob broke a photogate light beam. The time interval that the light was interrupted was recorded on an electronic timer attached to the photogate.
    Use the following data to complete a report.
    MAss of pendulum bob = 240.3 g
    Diameter of pendulum bob = 3.50 cm
    Initial height of pendulum bob = 48.0 cm
    Length of pendulum string = 2.14 m
    Time interval of photogate light interruption = 11.8 ms

    Your report should include the following:
    (a) conclusion as to whether or not the pendulum demonstrated the law of conservation of energy
    (b) calculations of the efficiency of the pendulum as a mechanical machine

    I am really confused how to even start this question. I think that I should figure out the Emechanical = Ek + eg

    =1/2(240.3)(0) + 240.3(9.8)(.48)

    to calculate the speed of when it hit, I did the following:

    The clue is the diameter of the bob
    it broke the beam for 11.8 *10^-3 seconds
    it is .035 meters in diameter
    so it went
    .035 meters in 11.8^10^-3 seconds
    which is about 2.97 meters/s

    From here, I don't really know where to go. Please help me.
  2. jcsd
  3. Jan 31, 2010 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You have made a good start. There is an issue with the units here; to get energy in Joules, you must use m, kg, and s for the other units. One of quantities in your Eg calculation is in the wrong units.

    You can now do the calculation of Etotal=Ek+Eg for when the pendulum is at the bottom of its swing.
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