gary's clocks


There is a lot of very good information on the web about pendulum theory.

This page tries to answer some of the questions specific to clock pendulums.

Some simple experiments are also outlined to help prove some of the concepts.

Typical info you are apt to find on the web about clocks

  • That a pendulums period depends only on the length of the pendulum and not the weight of the bob

  • A formula to compute the time for a pendulum to swing back and forth

    T= 2 pi*sqrt(l/g) where
    T= period in seconds for one complete cycle (a Tick and a Tock)
    g= acceleration due to gravity 32.2 ft/sec/sec
    l= length of pendulum in feet

  • If you had a pendulum that had no friction at the pivot or encountered no wind resistance it would oscilate back and forth forever. Regardless of the length of the cord or weight of the bob!

    Forever is a very long time

    We all know from experience that pendulums do not oscilate back and forth forever.

    The pendulums on our clocks would quickly come to rest if the weights were removed from our gear trains.

    The Escapement gives the pendulum a little push every Tick and tock.

    Without this impulse the pendulum and clock would stop.

    So here are some Clock pendulum questions for you

    Remember our goal is to find a pendulum that will oscilate back and forth with the least amount of help from the escapement wheel. Forever would be nice but we'd accept something like 50 oscilations before it stopped. Even 10 would do it.

  • Is it better to have a heavy Bob or a light one?
  • A heavy pendulum bob will oscilate for a much longer time than a light one. The reasons for this are similar to those for selecting the pendulum length. read on........

    Compare a heavy(10 pound) pendulum bob to a light one (1 pound)

    Two identical pendulums except for bob weight are released at once.
    The heavy one keeps going while the light one stops.

    comp pend weights

  • Should the pendulum be long or short?
  • Compare two pendulums where everything is the same except for the length.

    The short one requires a little push every once in a while to keep it going but the long one just keeps on going.

    two pends one long one short

  • Should the angle the pendulum travels between tick and tock be large (30 degrees) or small (2 degrees)?
  • Start a pendulum from a high angle.

    See that most of the energy is lost in the early stages of the cycle.

    This is similar once again to what happens on a playground swingset.

    It's real hard to keep the kid flying high but takes no effort to gently sway back and forth near the bottom of the arc.

    Makes sense that the pendulum arc be small too in our clocks.

    pend angles

    Compare Same pendulum with the force applied at the top and bottom

    If you were trying to keep the pendulum moving where would you push?

    Where do most of the clocks push on the pendulum to keep them in motion? (guess near the top) pendulum force compare

    upside down escapement

    Take advantage of pushing near the bottom of the pendulum

    uside down escapement

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