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This might be a very stupid question. But let me still ask. :) Perpetual motion is not possible. But by Newton's first law, a body remains in its state of motion unless an external force acts on it. Now, I wonder if the statement "perpetual motion is not possible" is true only in a practical...

Thanks Michael. That sounds convincing.

I agree. Just wondering, then does it not answer the question: "if there was acceleration toward the centre, then why doesn't the distance from the centre to the object reduce?"

Can you please explain what you meant by that relation? What I meant was, in circular motion, isn't the centripetal force canceled by centrifugal force, and hence there is no motion towards the center?

That's what he is trying to derive, without any assumptions. Because he then goes on to proving X <= 1 ft. Right? Intuitively, if we know " moving 3 balls by a distance X each is equivalent to moving 1 ball a distance 3X", we are not deriving the equation, we are using it, right?

Isn't the centripetal force balanced by a reaction force- centrfugal force?

Feynman's lecture on physics: chapter 4 derives equation for gravitational potential energy by a "lifting 3 balls" example. The book notes: "But the strange thing is that, in a certain way of speaking, we have not lifted two of them at all because, after all, there were balls on shelves 2 and...

In 2-5, an analogy is given for electromagnetic field: two corks in water, and the effect of jiggling one cork on the other (probably the up and down motion). Now, that sounded more like water is a medium propagating the energy. But the electromagnetic waves require no medium for propagation...