Pendulums are weights suspended from a fixed point along a chain or string. When the weight is pulled to one side and released, it begins to sway back and forth according to precise rules.
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Pendulums in Clocks
Because pendulum motion is so precise and the swinging of the pendulum tends to last a long time unless interrupted, pendulums are a classic mechanism for powering clocks without using electricity. However, the pendulum will stop eventually.
The classical explanation, offered by Aristotle, as to why a pendulum in motion will eventually stop was simply because all objects stop, eventually, if no force is applied to keep it moving. This explanation, however intuitive, turns out to be wrong.
Newton's Laws of Motion
The modern explanation is more complicated. According to modern physics, an object, once set in motion, remains in motion until an external force acts to stop that motion. A pendulum, completely isolated from all other forces, would continue to swing forever.
In reality, however, the pendulum does not swing in isolation from the rest of the universe. Since the swinging pendulum collides with air and the joint of the pendulum must scrape against the surface that supports it, it will gradually slow down as these interactions pull energy out of the pendulum. These forces are known as "friction," and they are what stops the pendulum.