Current Reading

This blog is primarily for me to blog my responses to books that I'm reading. Sometimes I blog about other stuff too, though.

Word cloud

Word cloud

Wednesday, July 20, 2016

Principia: Book 1, section 2, Proposition 4

This one was hard, because I'm pretty sure there's an error in the transcription.  The proof says that the proposition can be derived from Lemma 7 of the previous section, but I see no obvious way to get it from that.  OTOH, I can easily see how to get it from Lemma 11.

The first 7 corollaries are great.  The guy derives the inverse square law of gravity for circular orbits, assuming Kepler's law (T^2 ~ R^3).  Awesome.

Then Corollary 9 threw me through a loop.  The translators used the term "mean proportional" rather than "geometric mean."  But I sorted it out.

This book is a hard slog, but worth it.

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