Philosophiæ Naturalis Principia Mathematica
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The book that founded physics. Its opening chapter defines mass, momentum and force, and asks what it means to move at all: absolute and relative time, space and motion, with the spinning bucket and the two globes on a cord.
This is a sample: 1 of 5 chapters.
The fourth idea is the force the whole book is about: a pull towards a centre, which he calls centripetal. Gravity, magnetism, and whatever holds the planets on their curved paths are all of this kind.
On the second question, he refuses to define time, space, place and motion, which everyone thinks they know, but he warns that people know them only through the things they see, and that leads them astray. So he splits each into a true version and a measured one.
Newton begins with time, because time has the same problem. We measure time by days, by the sun going from noon to noon, and everybody treats those days as equal. But they are not quite equal, and astronomers have to correct them before they can calculate the motions of the planets accurately.
The causes are more helpful. True motion only ever changes when a force acts on the body itself.
The effects are the decisive clue, and here Newton describes an experiment he did himself. He hung a bucket of water from a long cord, twisted the cord many times, held the bucket still, and then let it go.
The cord between the two balls is pulled tight by their outward strain, so its tightness reveals how fast they spin, and a push on matching sides of both balls reveals which way, even with nothing else in sight.
Examples for: a curious adult with school physics who uses a smartphone every day; add one modern instance per idea, such as GPS satellites, a phone's accelerometer or a car braking
The lectures retell the English text of Project Gutenberg #76404, Andrew Motte's translation (first American edition, 1846). The author died in 1727; the work is in the public domain. The retelling, the drawings and the voice are made by AI and checked against the book by a second model; nothing was edited by hand. How AI is used.
The lectures are free to read, share and quote with a link to this page.
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