Randomly came across this article. http://en.wikipedia.org/wiki/Reactive_centrifugal_force

The diagram describes a different example but it makes a peng and lu kind of “energy” clearer, I think. The ball is tied to a string and spinning around a post. The string is exerting a centripetal inward force due to the circular motion. The ball is exerting reactive centrifugal force. The net force on the string is zero but the string is in tension. This seems similar to incoming force that is borrowed and rolled (back) with circular motion, creating the reactive centrifugal “peng” force on its own (so your arm for example “has no force”, meaning the net force appears to be zero, though some small force goes into creating the circular motion).

reactive centrifugal force

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