History · Depth 3 · Introductory · 1 min read
Isaac Newton
Isaac Newton (1643–1727) set out the laws of motion and the law of universal gravitation in his Principia, published in 1687.
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Who was Isaac Newton?
Isaac Newton (1643–1727) developed the laws of motion and the law of universal gravitation, which together explain everything from a thrown ball to the orbits of the planets.[1, 2]
The Principia
In 1687 Newton published the Principia, at the expense of his colleague Edmond Halley.[1] In it he set out three laws of motion:[1]
- An object stays at rest, or keeps moving at a constant speed in a straight line, unless an outside force changes that.
- The change in an object’s motion is proportional to, and in the direction of, the force acting on it.
- For every action there is an equal and opposite reaction.
Read Newton’s Laws of Motion for how they work.
Universal gravitation
Newton proposed that every mass attracts every other mass, with a force that weakens with the square of the distance between them.[2] He showed that such a force produces exactly the orbits Kepler had found, and he checked it against the Moon: its measured acceleration toward Earth, about 0.00272 m/s2, matched his prediction.[2] Read Gravity for the details.
Explaining orbits
To show how gravity could hold the Moon and planets in orbit, Newton imagined a cannon on a high mountain firing cannonballs ever faster, until one fell completely around Earth.[3] Almost three centuries later, satellites would do exactly that.
Real-life examples
Testing gravity on the Moon
Newton predicted how strongly the Moon, about 60 Earth-radii away, should accelerate toward Earth. The measured value, about 0.00272 m/s2, matched.[2]
The cannon on the mountain
To explain orbits, Newton imagined a cannon firing ever faster until the cannonball fell all the way around Earth.[3]
A friend who paid the printer
The Principia was published at the expense of Newton's colleague Edmond Halley.[1]
Connected across the map
- Newton's Laws of MotionThree rules that explain how every object moves: inertia, force equals mass times acceleration, and every action has an equal and opposite reaction.
- GravityThe attraction between all masses. Why it weakens with distance, why the Moon 'falls' around Earth, and how it creates tides and keeps planets in orbit.2 branches
- DerivativesThe derivative measures how fast something is changing at a single instant: the slope of a curve at one point, found as a limit of average rates of change.
- IntegralsAn integral adds up thin slices: the area under a curve as a limit of sums of rectangles, tied to derivatives by the Fundamental Theorem of Calculus.
- MomentumMomentum is mass times velocity. Why it is conserved in collisions, how an impulse changes it, and how airbags and rockets use it.
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- Astronomy 2e (free textbook) ↗
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Sources
Based on 3 sources from 1 institution: OpenStax.
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- ScholarlyOpenStax (Rice University)· Academic publisherAstronomy 2e, 3.2 Newton’s Great SynthesisOpened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0
- ScholarlyOpenStax (Rice University)· Academic publisherAstronomy 2e, 3.3 Newton’s Universal Law of GravitationOpened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0
- ScholarlyOpenStax (Rice University)· Academic publisherAstronomy 2e, 3.5 Motions of Satellites and SpacecraftOpened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0