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.
Lay your phone flat on the passenger seat of a car and open any free sensor app that shows the accelerometer. While the car rolls steadily along a straight road, the reading along the direction of travel stays near zero.
Newton gives each measure a home.
Time comes first. True, absolute time flows evenly of itself, without regard to anything outside, and its other name is duration. Relative, common time is any visible measure of that duration made by some motion, accurate or not, such as an hour, a day, a month or a year.
But we cannot see the parts of space or tell them apart, so we use measures we can see instead. We pick some body that we treat as unmoving, define every place by its distance and direction from that body, and judge every motion against those places.
Next, the causes. True motion is never created or changed except by a force pressed on the body itself. Relative motion can change with no force on the body at all: push the things it is compared with instead, and the relation changes.
First, a point about words. The relative quantities, our hours, our measured places, our speeds against the ground, are not the true quantities they are named after, but the visible measures we commonly use instead, accurate or not.
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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