The Dummy Version of Pendulum Movement

Simple pendulum period frequency

Have you ever read Edgar Allan Poe's "The Pit and the Pendulum"? Now there was a writer who knew how to do the scary stuff: it's small wonder that so many wannabe anonymous scary types wear Poe masks (c.f., "The Following"). Well, based on his short story we can tell that good ol' Ed knew at least a little bit about how pendulums work, which made the story all the scarier (that and the rats...). By analogy, however, we'd have to assume that a similar story that was written by eHow's Alexis Kezirian wouldn't be scary, it'd probably be a comedy, given that she seems to know jack about pendulums; as she so amply demonstrated in "How to Decrease the Period of a Pendulum" (now deleted by Leaf Group)¹.

Oh, Alexis started off all right, given that she presented both a definition of "period" in this particular context and a formula for determining the period in the (DMS-mandated) introduction:

"The period of a pendulum is defined by the time it takes the object to travel the entire course in both directions. In order to shorten the period of a pendulum, it is helpful to first understand the mathematical equation relating the period (T) to the length (L) of the pendulum itself: T = 2 pi (L/9.8)^(1/2), where 9.8 represents the gravitational force on Earth. "

Clearly, Kezirian found a source with knowledge of pendula, although that "represents the gravitational force on Earth" bit is a rather clumsy rewording of "g"; not to mention that she neither included the units of g nor whether the number is in the metric system (it is) or in imperial units. Whatever. Now, let's see what she said will shorten a pendulum's period. First, there's

"Decrease Pendulum Length: Look at the equation relating the period to the length of the pendulum string. Notice that these two factors are directly proportional to each other. Shorten the length of the string or material from which the object dangles and swings. Keep the object, pivotal point and medium the same as they were before attempting to shorten the period." And there you have it: that's how you decrease the period of a pendulum: you shorten the rod. Sure, Alexis could have said that in three words instead of sixty, but then she wouldn't have met the site's minimum word count (300 to 500). Unfortunately, that same minimum word count apparently prevented Kezirian from quitting while she was ahead. We presume that's why she kept writing, saying

"Increase the Frequency: Recall the equation: T = 1/f. Mathematically, this proves that a higher frequency causes a shorter period.

Push the object from its maximum amplitude with additional force in order to raise the frequency of the swinging pendulum.

Note that accelerating the object may result in an amplitude that differs from the initial circumstance, but that amplitude has no effect on the period of a pendulum."

Oh. My, Word... this woman actually thought that shoving the bob of a pendulum "harder" increases its frequency, Clearly, the moron didn't understand the first of her equations, in which the only force component is g, the gravitational constant (Kezirian probably didn't even realize g is a force). Shoving the mass at the end of the pendulum merely increases the amplitude of the pendulum's swing, which has no effect on its period -- which Alexis just said, her own darned self!

No the only other way to decrease a pendulum's period is to move the whole works to a stronger gravitational field -- the pendulum would swing faster on Jupiter, for instance. But Alexis not only didn't know that, she couldn't figure it out from the equation she copied (sort of) and pasted. Bad Alexis! For her work, she is the proud winner of a shiny Dumbass of the Day award!

¹ The original URL, ehow.com/how_8419547_decrease-period-pendulum.html, can still be viewed at archive.org.

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