[Split Thread] Pendular Motion (And Relativity)

This thread is split from The Theory of Relativity thread.

Posted By:Loss Leader

There is a thread on thenakedscientists forum about pendulum and conservation of energy.
The issue is rotational kinetic energy that 'apparently breaks' relativity.

There are many smart people on that forum but nobody is defending relativity.
What's going on? How to debunk the claim?
Some healthy skepticism is needed.

via International Skeptics Forum https://ift.tt/2uJiomS