Great IdeasNature and science
Motion and force
Why things move, fall and orbit.
The question
Why do things move, fall and keep moving? Does motion need a push to keep going?
- 1 Aristotle 350 BC
- 2 Archimedes 250 BC
- 3 Thomas Aquinas 1268
- 4 Johannes Kepler 1620
- 5 Galileo Galilei 1638
- 6 Isaac Newton 1687
- 7 Michael Faraday 1832
- 8 Albert Einstein 1916
- Motion needs a mover to keep going
- Motion keeps going unless something changes it
How the books answer
Oldest first. Each stop is our own short summary of what the author argued; follow the link to the rung for the whole lesson.
c. 350–322 BC
Aristotle, Physics
Motion needs a mover to keep going
Aristotle teaches that whatever moves is moved by something. Heavy things fall toward their natural place, and he said heavier things fall faster. He also argued against empty space, noticing that in a void nothing would explain why a moving thing should ever stop.
“Further, no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here?”
3rd century BC
Archimedes, On the Equilibrium of Planes, On Floating Bodies, Measurement of a Circle, The Sand-Reckoner, On the Sphere and Cylinder, and others
Archimedes studies bodies in balance rather than in motion. He proves the law of the lever and finds the centers of gravity of shapes. His mathematical mechanics became a model for Galileo.
“Two magnitudes, whether commensurable or incommensurable, balance at distances reciprocally proportional to the magnitudes.”
1265–1274 (unfinished)
Thomas Aquinas, Summa Theologica, First Part
Motion needs a mover to keep going
Aquinas builds on Aristotle's idea that everything moved is moved by another. His first way of arguing for God reasons that the chain of movers can't go back forever, so there must be a first mover. Readers still discuss whether the argument depends on Aristotle's physics.
“It is certain, and evident to our senses, that in the world some things are in motion.”
1617–1621 (Books IV–V, 1620–1621)
Johannes Kepler, Epitome of Copernican Astronomy, Books IV and V
Motion needs a mover to keep going
Kepler asks what physically moves the planets. He imagines a force from the turning sun that sweeps them along, weaker farther away. He was still thinking of a mover, but now as a physical force that could be studied with mathematics.
1638
Galileo Galilei, Dialogues Concerning the Two New Sciences
Motion keeps going unless something changes it
Galileo shows by reasoning and by rolling balls down ramps that, without air resistance, heavy and light bodies fall together, speeding up evenly. A ball on a level surface with no friction would roll on and on. A thrown ball follows a curve called a parabola.
1687
Isaac Newton, Mathematical Principles of Natural Philosophy
Motion keeps going unless something changes it
Newton states the three laws of motion. A body keeps its rest or straight motion unless a force acts; force changes motion; every action has an equal and opposite reaction. With his law of gravity, these explain motion on earth and in the heavens.
“Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impress'd thereon.”
1832–1855
Michael Faraday, Experimental Researches in Electricity
Faraday finds that moving a magnet near a coil of wire makes an electric current, and that a current can make a wire move near a magnet. Motion, electricity and magnetism are linked. His discoveries led to electric motors and generators.
“Being left in, the needle resumed its first position, and then the magnet being withdrawn the needle was deflected in the opposite direction.”
Go to the rung: Faraday: Experimental Researches in Electricity
1916 (English 1920)
Albert Einstein, Relativity: The Special and the General Theory
Einstein asks what motion means when everything is moving relative to something else. If the speed of light is the same for every observer, then time and length must change with motion. Newton's laws still work at everyday speeds.
“Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity).”
For young readers
Roll a ball across the floor. It slows down and stops. Long ago, people thought this meant things stop moving when nothing pushes them.
Galileo and Newton saw it differently: the ball stops because the floor rubs against it (friction). On smooth ice it rolls much farther. Out in space, with nothing to slow it, it would roll forever! Try rolling a toy car on carpet, on wood and on a smooth tray to see the difference.
Talk about it
- Drop a heavy book and a light ball at the same time. Which lands first? Try it!
- Why do you lean forward when a car stops suddenly?
- Aristotle's ideas were wrong in some ways but lasted 2,000 years. Why do you think?
- Newton's third law says every push has a push back. Where do you see that when you jump?
Where else on the site
- Newton's First Law: Inertia
- Free Fall
- Newton's Second Law: F = ma
- Newton's Third Law
- Inclined Planes
- Levers and Balance
Every rung tagged with this idea:
These pages are our own short synthesis, written for families. They were inspired by Mortimer J. Adler's Syntopicon, the index of great ideas in volumes 1 and 2 of Great Books of the Western World, but the questions, summaries and groupings are ours, not the Syntopicon's. Short excerpts come from public-domain translations, credited where they appear.