I have very little knowledge of physics but don’t fully understand how a weight can serve to power a pendulum clock without verging on something like perpetual motion, which I know is of course impossible. How can a pendulum keep swinging almost indefinitely like that? And if a pendulum can use gravity to power itself, why can’t we use gravity to power larger scale devices?
In: Physics
The pendulum itself is just for keeping time; it isn’t the main energy-storage medium for the clock. You’re right in thinking that, if that were all there was to it, the clock would not-so-gradually grind to a halt. After all, a free-swinging pendulum eventually slows down, and it doesn’t have to spend energy driving clockwork.
Instead, there’s a sort of mechanical battery that periodically recharges the bob’s swing. This could be a spring that’s wound daily and, every, say, five minutes is allowed to click one notch out and jolt the pendulum. Or, in some elegant grandfather clocks, there’s a heavy weight that, by ratcheting down a fraction of an inch, boosts the pendulum. When the weight has, bit by bit, fallen to the bottom of its track, it needs to be lifted to the top to recharge.
It doesn’t swing indefinitely. It essentially winds a coiled spring, which is released in small amounts by the escapement , and that coiled spring drives the movement itself. As the energy produced depletes, the pendulum (or weights) must be set in motion periodically. It is much more complicated, but, that is the basic idea
The pendulum lowers down just a tiny bit every swing, just enough to keep it going. Every so often (day to few days) the user needs to wind it back up, which brings the pendulum upwards and ready to slowly fall again.
I’ve also seen some pendulum clocks that are battery powered, supplying just enough energy to keep the pendulum swinging until the battery runs out.
We can use gravity to power devices and do. A common one is to pump water uphill, then harvest that power when the water comes back down.
As to the clock, the weights don’t work forever. The weight pulls on the pendulum to keep it swinging, and as it does the weight falls down. Eventually someone has to come along and lift the weight back up, otherwise the clock will eventually run out of power.
The weight is connected to a string which unwinds and pulls on a gear as it falls down and this powers the rest of the clock.
When the weight is at the bottom, you have to lift it back up or else it won’t pull any more and the clock won’t go.
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