"Modern" physics introduced B. C. E.

[Loren Booda]

Think of bringing back to the civilizations before the common era physics that could practically be used. For instance, an electric magneto might be fashioned by ancient peoples with access to magnetite and copper. Using his laws, planetary motion could easily be forcast accurately long before Kepler. They might be able to isolate radioactive compounds. Even a crystal radio transmitter and receiver would be constructible then.

What modern physics would you share with the people of the past that they might implement?

[selfAdjoint]

Theoretical: Newton's three laws, with the conclusion about centripetal force. Never mind gravity, this would enable them to design machines and analyze mundane occurrences.

Practical: Screw cutting. I mean the accurate way with lapping that has been described in the amateur scientist column. This is the breakthrough into fine manufacturing.