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Homework Help: Calculating Gravity with Pendulum - Factors of influence

  1. May 22, 2013 #1
    Sorry if I fill any of this out incorrectly, this is my first post.
    1. The problem statement, all variables and given/known data
    Basically, we are told to calculate gravity with the use of a pendulum. Now, this part I understand, alter the length, record the period at each length, calculate averages and substitute these averages into the formula, T = 2pi sqrt (L/g)

    2. Relevant equations
    As stated above.

    3. The attempt at a solution
    I have obtained my average for gravity, which is 9.53m/s^2. But I need some clarification on a few minor things.

    In my scientific discussion, I have said that:
    -human reaction time
    -Air resistance
    -Weight Distribution (of the mass on the end of the pendulum)

    All play a part in the result for gravity not being 9.8, however there are a few things that I am tempted to include, but not too sure if they are correct.

    1. The very, very, very small impact that the gravitation force between the metal mass on the end of the pendulum and the metal retort stand used to hold the pendulum has. I.e, this small gravitation field is slightly altering the course of the pendulum and as such slightly altering the period time.

    2. The elasticity of the string (fishing line) used on the pendulum. Wouldn't there be a very small amount of elasticity which would be altering the length of the string throughout the course of its period?

    Any help would be really appreciated.
  2. jcsd
  3. May 22, 2013 #2


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    Remember the formula you mentioned to find out time period is only applicable for Pendulum performing Simple Harmonic Motion, that is, by undergoing (infinitesimally) small displacement. So, for Macro-displacements, the motion is no longer SHM and the formula is not very accurate.
  4. May 23, 2013 #3
    I would assume that the reason free fall acceleration is so deviated is that the experiment was performed using large angles of swinging, up to about 15 degrees should have been good enough.

    You could include that the length of the pendulum may not have been measured precisely, also the uncertainty in your timekeeping device.

    Also, the larger angle you use the faster speed pendulum bob obtains, since the force of air resistance is proportional to the square of the speed, that influenced the period slightly.
  5. May 23, 2013 #4


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    Much too small to be of interest.
  6. May 23, 2013 #5


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    How did you measure period? Did you time one swing, or time ten swings and divide by ten? If you used multiple swings, mis-counting happens sometimes.
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