It’s all downhill but sometimes by interesting routes.

NOTES

Back to the pendulum. Just because it's simple; as we look close, it might not be as simple as it seemed at first. It's not just the pendulum bob that moves - the support is moving too, and the object that the support is connected to is probably moving also - and the momentum of the support is in the opposite direction of the momentum of the bob, so the total system has zero momentum throughout the cycle of the pendulum's motion.

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