A pendulum works by converting energy back and forth, a bit like a rollercoaster ride. When the bob is highest (furthest from the ground), it has maximum stored energy (potential energy). The key parts of a pendulum clock are:
1. A dial and hands that indicate the time.
2. A weight that stores (potential) energy and releases it to the clock mechanism as it falls, very gradually, over the course of a day (or several days, if you're lucky). Winding the clock raises the weight back up, storing more potential energy to power the mechanism.
3. A set of power gears that take energy from the falling weight and use it to drive the clock mechanism at the correct speed. If we use a really heavy weight and the right power gears, the weight will store enough energy to drive the clock for days without us having to wind it up. (Remember the law of conservation of energy here: the longer the clock runs, the more energy it uses; a clock with a heavier weight can store more potential energy so, generally speaking, it's going to run for longer without winding than one with a lighter weight.)
4. A set of timekeeping gears that drive the different hands around the clockface at different speeds. These are usually finer and more precisely made than the power gears.
5. A pendulum and escapement that regulate the speed of the clock and keep it (more or less) constant.
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