How Can the Period of a Pendulum Be Increased?
In Edgar Allen Poe's famously frightening short story, "The Pit and The Pendulum," the author builds suspense using the swinging movement of a giant blade at the end of a chain as it moves closer and closer to the narrator, threatening to slice the man in half. Timekeeping, regularity and the motion of a pendulum are all central to this tale of horror, providing a handy physical science lesson: Never lie underneath a giant swinging blade.
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What is a Simple Pendulum?
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Imagine a hypnotist with a watch on the end of a chain, slowly swinging it back and forth in front of his volunteer. Steadily, it swings back and forth, back and forth, until the volunteers eyes glaze over. The watch on a chain is an example of a simple pendulum. A weight, or bob, is suspended at the end of an arm of some kind, whether flexible or rigid. The other end of the arm is a fixed point.
Measuring the Period
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The period of a pendulum is the amount of time it takes for the pendulum to pass through one complete circuit. In this case, a circuit is defined as returning to its starting point. A pendulum begins a circuit when it is pulled to one side to begin its swing. Pull the bob to the right, and gravity will pull it right back down toward the ground. It won't simply stop at the bottom, though. It will continue in an arc until it reaches a point on the left, at roughly the same height as it was on the right. Add the return trip all the way back to the right to that, and the total time is called the period, and is usually notated (T).
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Changing the Period
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You can try to change the period in several ways, but only one way will work. If you add more mass to the bob to make it heavier, it will not change the amount of time it takes the pendulum to swing. If you pull the bob to a higher starting point, its path will be longer, but it will swing faster to make up for it. The only way to change the period of a pendulum is to shorten the arm on which it is swinging. The shorter the arm, the shorter the period. Nothing else will work on a free-swinging pendulum.
Uses
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Besides being handy for a hypnotist, pendulums can be found many places. A child on a swing at the park is a pendulum. A grandfather clock uses a form of pendulum, but the "fixed" end is attached to a gear mechanism that requires occasional winding. Otherwise, the pendulum would slow to a stop and need to be restarted much more frequently. The unwinding of the spring transfers stored energy to the pendulum to counteract the friction that would slow it down.
Fun Facts
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Some people believe that a pendulum can be used as a dowsing tool to find water or mineral sources underground. Pendulums are sometimes used by psychics, or by people who wish to predict the sex of a baby in utero. In some medical communities in Europe, pendulums are used for diagnostic purposes.
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References
- Photo Credit sylvar/Flickr.com
