A pendulum has a specialized form of motion. In standard form, it can be an accurate timekeeper and that's made it important to clockmakers. The swinging movement also can be seen in other objects. The metronome uses the same motion to set a musical beat. In addition to timing, a pendulum's swing has momentum and energy. Crime lab technicians use a ballistic pendulum to test firearms, and a wrecking ball's force will bring down a building.
A mechanical clock uses a pendulum to keep accurate time. The time of the pendulum's swing, called the period, depends on the force of gravity and the pendulum's length. The top end of the pendulum's arm connects to a mechanism that drives a gear system. The gears drive the hands of the clock. A little bit of the pendulum's motion is lost to friction; this is made up by a wind-up spring or weights.
A Foucault pendulum can, by itself, be used to tell time. It's typically made of a heavy metal ball attached to a very long wire. The wire is hung from a high point in such a way that the pendulum is free to swing in any vertical plane. When the ball is carefully released, it swings back and forth, but over time, the turning of the Earth changes the swing's direction. At the poles, the pendulum will cover a full circle on the ground in one day. At the equator, the Earth doesn't affect it; it will always swing in the same place. At places in-between, it will cover part of a circle in one day, increasing with latitude. If you know the latitude, the pendulum's position will reveal the time.
Used to demolish buildings, a wrecking ball is another example of pendulum motion. A skilled crane operator swings the wrecking ball on a strong cable, aiming it at the building to be taken down. Energy is stored in the upswing, and is released when the ball hits something.
The swinging motion used to bowl also illustrates how the partial swing of a pendulum stores energy. You store energy in your arm and the ball on the backswing. It's a result of lifting the weight of the ball against gravity. The energy is released when you let the ball go, and becomes its forward motion into the bowling lane.
Used by police departments for many years, a ballistic pendulum consists of a large block of wood hung on cords. The mass of the wood is precisely known. A technician fires a bullet into the block. The bullet lodges into it, setting it in motion. The farthest point of its backward swing indicates the bullet's momentum and energy. The technician can then determine the bullet's velocity, given the bullet's mass.
A mechanical metronome uses a pendulum's swing to keep musical time. It has an adjustable bob that can slide up and down a solid arm. It's arranged upside-down compared to a clock pendulum; when the bob is highest, the swing period is longest.
- Photo Credit sylvar, creativecommons.org
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