The history
From Aristotle's pushing air to a kilogram set by the Planck constant: 2,400 years of learning why things keep moving.
For two thousand years people believed that moving things need a constant push. Philoponus, Ibn Sina and Buridan chipped away at that idea, Galileo rolled balls to show that only friction stops motion, Kepler named inertia, and Newton made mass its measure. Since then the kilogram has gone from a metal cylinder to a constant of nature, astronauts have learned to weigh themselves by wobbling, and satellites have shown that inertia and gravity agree to 1 part in 10¹⁵.
1618–21
The word 'inertia' in physics
Johannes Kepler, Linz
1644
Inertia as straight-line motion
René Descartes, Amsterdam
1687
Mass defined as quantity of matter
Isaac Newton, London
1889
International Prototype of the Kilogram
1st CGPM, Sèvres
1973
Body mass measured in orbit
Skylab, NASA
2019
Kilogram defined by a constant of nature
CGPM, via the Planck constant
400 BCE – 1500
Aristotle says motion needs a mover; Philoponus, Ibn Sina and Buridan give the moving body its own impetus.
350 BCE
c. 350 BCE
In his Physics, Aristotle taught that a moving thing keeps moving only while something pushes it. A thrown stone, he suggested, is kept going by the air rushing in behind it. Everyday life seemed to agree: carts stop when the ox stops pulling.
Why it mattered. It set the puzzle for 2,000 years: why does anything keep moving at all?
530
c. 530 CE
Philoponus attacked Aristotle's air-pushing idea. He argued that the thrower gives the stone an inner 'impressed force' that carries it on and slowly wears away, and that a stone would move even through empty space.
Why it mattered. It moved the cause of motion from the air into the moving object itself.
1020
c. 1020
In The Book of Healing, Ibn Sina described an inclination given to a projectile that is used up only by the resistance of the air. In a vacuum, he reasoned, nothing would use it up, so the motion would not stop.
Why it mattered. It was one of the first statements that only resistance makes moving things stop.
1350
c. 1350
Buridan said a thrown body carries an 'impetus' that stays with it until air resistance and gravity destroy it. The impetus was bigger for more matter and more speed, rather like what we now call momentum. It explained why a heavy millstone is hard to stop once it spins.
Why it mattered. Impetus that doesn't fade on its own is only a short step from inertia.
1590 – 1700
Galileo experiments, Kepler names it, Descartes straightens it, and Newton makes mass its measure.
1604
c. 1604
Galileo rolled bronze balls down one ramp and up another. The ball climbed back to nearly its starting height, however gentle the second slope. Make the second ramp flat, he reasoned, and the ball would roll on for ever: it is friction that stops it.
Why it mattered. It was the first experiment to show that motion carries on unless something takes it away.
1618
1618–1621
In his Epitome of Copernican Astronomy, published in parts from 1618 to 1621, Kepler used the Latin word inertia, 'laziness' or 'idleness', for matter's resistance to being moved. His inertia made things tend to come to rest; the modern meaning came later.
Why it mattered. The idea finally had a name.
1632
In his Dialogue on the Two Chief World Systems, Galileo argued that a stone dropped from a ship's mast lands at the foot of the mast even while the ship sails on, because the stone keeps the ship's forward motion as it falls.
Why it mattered. It explained why we don't feel the Earth moving: everything shares its motion.
1638
In his last book, published in Leiden while he was under house arrest, Galileo stated that a body moving on a level surface keeps its speed as long as nothing gets in its way. For him 'level' followed the curve of the Earth, so his inertia was still slightly circular.
Why it mattered. Inertia became a principle of physics, backed by measurement.
1644
In his Principles of Philosophy, Descartes set out laws of nature: each thing stays in the same state unless something changes it, and every moving thing tends to go on in a straight line.
Why it mattered. He was the first to make inertia straight, as Newton would.
1687
5 July 1687
Newton's Principia defined 'quantity of matter', our mass, and the 'vis inertiae', the power of every body to resist changes to its motion. His first law says every body stays at rest or in steady straight-line motion unless a force acts, and his second makes mass the measure of that resistance: F = m a.
Why it mattered. Inertia and mass became the foundation of all of mechanics.
1700 – 1910
The moment of inertia, the gyroscope, the platinum kilogram and Eötvös's torsion balance.
1765
In his book on the motion of rigid bodies, Euler named the 'moment of inertia', the spinning version of mass, and showed how it governs how bodies turn, wobble and tumble.
Why it mattered. It explained flywheels, spinning tops and skaters.
1852
Foucault spun a heavy wheel in a set of rings and watched its axis hold its direction in space while the Earth turned underneath. He called it a gyroscope, 'to see the rotation'.
Why it mattered. Rotational inertia became a tool, now used in compasses, ships and spacecraft.
1889
September 1889
The conference approved the International Prototype of the Kilogram: a cylinder of 90% platinum and 10% iridium, 39 mm high and 39 mm across, kept in a vault near Paris. For 130 years a kilogram was, by definition, its mass.
Why it mattered. Every scale in the world was traceable to one metal cylinder.
1889
1889–1908
With a delicate torsion balance, Eötvös checked whether materials with the same weight also have the same inertia. By 1890 he found no difference to about 5 parts in 100 million; long runs with Pekár and Fekete from 1906 to 1908, published in 1922, pushed that to a few parts in a billion.
Why it mattered. It showed inertial and gravitational mass are the same to astonishing accuracy.
1900 – today
Einstein builds on the equivalence of the two masses, astronauts weigh themselves by inertia, and the kilogram is set by nature.
1907
Einstein realised that someone falling freely feels no weight at all. If inertial and gravitational mass are truly equal, gravity and acceleration cannot be told apart. He made this equivalence principle the starting point of general relativity, finished in 1915.
Why it mattered. A coincidence Newton had just accepted became the key to a new theory of gravity.
1971
2 August 1971
Standing on the airless Moon, astronaut David Scott dropped a geologist's hammer and a falcon feather together. They hit the ground at the same moment, just as Galileo had said.
Why it mattered. Millions watched gravitational and inertial mass agree live on television.
1973
Scales can't work in orbit, so Skylab's crews sat in a chair on springs and let it wobble. A heavier astronaut swings more slowly: the chair's springs had k = 606 N/m and the empty chair's period was 0.848 s. It measured body mass to about 0.1%.
Why it mattered. It weighed people by their inertia alone.
2005
During Expedition 11, the Space Linear Acceleration Mass Measurement Device was installed on the ISS. Springs pull the astronaut's guide arm with a known force, a sensor measures the acceleration, and a laptop works out m = F ÷ a, from 43 to 109 kg, to about 0.2 kg.
Why it mattered. Newton's second law became a bathroom scale for space.
2019
20 May 2019
After a vote in November 2018, the kilogram stopped being the mass of a metal cylinder. It is now fixed by the Planck constant, h = 6.626 070 15 × 10⁻³⁴ J·s, and realised with instruments such as the Kibble balance, proposed by Bryan Kibble in 1975.
Why it mattered. Anyone, anywhere, can now make a kilogram from a constant of nature.
2022
14 September 2022
The MICROSCOPE satellite (2016–2018) kept titanium and platinum test masses floating inside each other as they fell round the Earth. Its final result found no difference in how they fall: η = (−1.5 ± 2.7) × 10⁻¹⁵.
Why it mattered. It is the most precise test yet that inertial and gravitational mass are the same.
By the numbers
The smallest difference each experiment could have detected in how different materials fall. Lower is better; none has ever found one.
Did you know?
Kepler's word inertia is Latin for idleness or laziness. He thought matter was lazy about moving; we now know it is just as lazy about stopping.
The platinum-iridium kilogram was only 39 mm tall. Its official copies drifted by about 50 micrograms over a century, one reason it was retired in 2019.
Gravity at the International Space Station's height is still about 90% as strong as on the ground. Astronauts float because they and the station are falling round the Earth together.
MICROSCOPE's 1 part in 10¹⁵ is like weighing a 100,000 tonne ship and noticing an eyelash.
The fastest recorded figure-skating spin is about 308 rpm, over 5 turns a second, all from pulling the arms in.
The people
384 – 322 BCE · Philosopher · Greece
His rule that motion needs a mover lasted nearly two thousand years.
c. 490 – c. 570 · Philosopher · Egypt
First to say the thrower gives the stone an inner force of its own.
c. 1301 – c. 1360 · Philosopher · France
His impetus, which only resistance destroys, came close to momentum.
1564 – 1642 · Physicist and astronomer · Italy
Showed with rolling balls that only friction stops motion.
1571 – 1630 · Astronomer · Germany
Gave inertia its name.
1596 – 1650 · Philosopher and mathematician · France
First to state that moving things go on in straight lines.
1643 – 1727 · Physicist and mathematician · England
Made inertia his first law and mass its measure.
1707 – 1783 · Mathematician · Switzerland
Named the moment of inertia and worked out how bodies spin.
1848 – 1919 · Physicist · Hungary
Showed inertial and gravitational mass agree to parts per billion.
1879 – 1955 · Physicist · Germany
Turned the equality of the two masses into general relativity.
1929 – 2021 · Physician and astronaut · USA
Led the team that built Skylab's wobble-chair mass meter.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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