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Homework Help: Mechanical Energy in a Pendulum

  1. May 22, 2007 #1
    1. The problem statement, all variables and given/known data

    Hi,I am taking a grade 11 physics correspondance course and I have been stumped for the last few weeks on what they are asking in a particular question.

    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
    (c) any statements of sources of error
    (d) your suggestions for improving the design of the investigation.
    Relevant equations[/b]

    3. The attempt at a solution

    Am I supposed to perform this experiement myself, becuase they are giving me all the measurments (therefore am I just doing an analysis) I am not quite sure how to even begin attempting this question. Any suggestions would be very appreciated .
    Thank you
    Pharm 89
  2. jcsd
  3. May 23, 2007 #2


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    Homework Helper

    The swinging mass interrupts the beam of the photogate as it swings through the gate at the lowest point. The speed of the mass at the lowest point can then be calculated with

    [tex]v_L = \frac{d}{t}[/tex]

    where [tex]d,\ t[/tex] is the diameter of the mass and the time of interruption.
  4. May 23, 2007 #3
    i'll try to attempt this..... but let me tell you that i'm not really that good at physics.

    i think you will have to attempt the question only.... coz you won't be having the materials,,, especially the photothing

    and anyway, it's a report that you will have to write on the experiment. this will include (a) (b) (c) (d)

    (a) i guess that to demonstrate law of conservation of energy, the K.E. at the lowest point, should be equal to the P.E. at initial point. but naturally i don't think it K.E. = P.E., due to friction, etc.....
    note: Law of conserv. of energy will be obeyed if you consider the whole system: the pendulum+air around+ any other thing around(duh!!!0)

    (b) efficiency would be the Power out divided by the Power in, something like that... guess its the final power over the initial power, but remember that efficiency is always between 0 and 1

    (c) could be parallax error(the favourite one!!!), especially with the metre stick, the angle from which the bob will be realeased at first should not be too large because of something i just did at school bt didn't understand very much as usual!!! also the bob should be swinging in one plane only (hope you get wot i mean) and should not rotate on itself, and at most you could say that the measuring instruments are still not precise enough.

    (d) Carry the exp 2 more times, making averages, use of graphs,
    uummm i've never worked with the photothing, so i wouldn't be able to tell you any improvement about it.....

    hope i helped you at least a little!!!...lol
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