99 Fun Facts About Newton's Laws of Motion

What closed Cambridge University in 1665, sending Isaac Newton home for two famously productive years?

The Great Plague shut the university, and in less than two years at home Newton began the work on calculus, light and gravity that made his name.

A costly book on what stopped the Royal Society funding Newton's Principia?

Fish: the heavily illustrated De Historia Piscium sold so poorly that the Society could not pay for Newton's book, and it later settled one official's salary in unsold copies.

Newton's first law says a body's natural behaviour is to move how?

In a straight line at constant speed: on a perfectly frictionless surface a sliding puck would simply never stop. Friction and air drag are what halt things on Earth.

According to Newton's first law, what is needed to change a body's state of rest or uniform motion?

A force impressed upon it, in Newton's own words; and a change of direction counts, so a car rounding a bend at steady speed is being forced the whole way round.

When do bathroom scales in an elevator read more than your usual weight?

As it starts upward: by Newton's second law the floor must push up harder than gravity pulls down to get you moving, and the scales read that push. At a steady speed the reading drops back to normal.

In modern terms, the net force on a body equals its mass multiplied by what?

Its acceleration gives the famous F = ma, a formula that appears nowhere in Newton's Principia; it was in print by 1716, in Jakob Hermann's textbook Phoronomia.

When the net force on a body is zero, what is it said to be in?

Mechanical equilibrium does not mean motionless: a parked car and one cruising dead straight at steady speed both qualify, because in each case the forces balance.

Newton's third law states that to every action there is always opposed what?

An equal reaction, but the catch students miss is that the two forces act on different bodies, so they never simply cancel.

Newton's third law is related to which more fundamental principle?

Conservation of momentum: in any push or collision, whatever momentum one body gains the other loses, so the total never changes.

In Newton's thought experiment, what does a cannonball fired fast enough from a mountaintop do instead of landing?

It circles the Earth: the ball never stops falling, but the ground curves away beneath it just as fast, which is how the Moon and every satellite stay up.

Linear momentum is the product of mass and what?

Velocity, not plain speed, so the heading matters: two identical cars meeting head-on at the same speed have a combined momentum of exactly zero.

In SI units, momentum is measured in what?

Kilogram metres per second is simply mass times velocity spelled out; the same unit can be written as the newton second, a force multiplied by the time it acts.

Which ancient thinker taught that moving objects stop unless something keeps pushing them, a view that lasted nearly two thousand years?

Aristotle was describing what everyone sees: stop pushing a cart and it halts. Galileo and then Newton turned that around, arguing it is the stopping, not the moving, that needs a cause.

One newton accelerates a mass of one kilogram at what rate?

One metre per second squared: the unit was built from the second law itself, so one newton, one kilogram and that acceleration fit together with no conversion factor.

On Earth, about how many newtons does a one-kilogram bag of sugar weigh?

About 10: gravity pulls on every kilogram with 9.8 newtons. Bathroom scales really measure that force and merely display it as kilograms.

Force is what kind of quantity?

A vector: a force has a direction as well as a size, which is why two equal forces pulling one object in opposite directions add to nothing, as in a deadlocked tug of war.

How does an airbag cut the force on a passenger in a crash?

It lengthens the stop: the passenger sheds the same momentum with or without the bag, but spread over a longer time the force is far smaller. Crumple zones work the same way.

Galileo worked out the idea behind Newton's first law by rolling balls along what?

Inclined planes: a ball let go on one ramp climbs a facing ramp to nearly its starting height, so Galileo reasoned that on a level track it would simply roll on forever.

The word 'inertia' derives from a Latin word meaning what?

Idleness: Kepler, who brought the word into physics, took rest to be the natural state, so inertia to him was a body's laziness about moving at all.

An astronaut on the Moon weighs a sixth of their Earth weight, but what happens to their mass?

It stays the same: mass is the amount of matter in a body, while weight is only how hard the local gravity pulls on it, and lunar gravity is about a sixth as strong.

Inertial mass measures an object's resistance to what?

Being accelerated by a force: the more mass, the less a given push changes the motion, which is why a loaded trolley is so much harder to get going, and to stop, than an empty one.

A rocket engine produces thrust in accordance with which of Newton's laws?

The third law: the engine hurls hot gas backwards and the gas shoves the rocket forwards. It needs nothing behind it to push against, which is why rockets work best in a vacuum.

Friction is the force resisting what?

Relative motion of surfaces in contact is what it resists, and it usually grips hardest before anything slides: once a heavy crate breaks free, it takes less push to keep it moving than it took to start.

Who discovered the classic laws of sliding friction around 1493, but never published them?

Leonardo da Vinci scribbled them in a pocket notebook under a sketch of an old woman, and a museum director later dismissed the jottings as 'irrelevant notes'. The laws were found afresh two centuries on.

Which planet was found in 1846 only after mathematicians used Newton's laws to predict where it must be?

Neptune: Uranus kept straying from its predicted path, so Urbain Le Verrier calculated where an unseen planet must be tugging at it, and a Berlin astronomer found it on his first night of looking.

Which detail of Newton's apple story was added by later storytellers?

It hit him on the head is the later embellishment. In 1726 Newton told his friend William Stukeley, over tea under some apple trees, only that a falling apple set him wondering why things always drop straight down.

When an Apollo astronaut dropped a hammer and a feather on the Moon, what happened?

They landed together: with no air to hold it back, the falcon feather fell as fast as the geology hammer, just as Galileo had reasoned. David Scott did it live on television in 1971.

According to a possibly apocryphal tale, Galileo dropped objects from which landmark?

The Leaning Tower of Pisa story rests on one man's word: Vincenzo Viviani, the young pupil who worked beside the blind Galileo in his last years and became his first biographer.

Why was Galileo's last great book, on motion, printed in Holland and not Italy?

The Inquisition had banned him from publishing anything, future works included, so Two New Sciences was spirited out of Italy and printed in Leiden in 1638. By then Galileo was blind and under house arrest.

Who encouraged and helped fund the publication of Newton's Principia?

Edmond Halley paid for the printing himself, then used Newton's laws to predict that the comet of 1682 would come back in 1758. It did, and it has carried his name ever since.

In which language was the Principia written?

Latin was its only language in Newton's lifetime: no English translation appeared until Andrew Motte's in 1729, two years after the author's death.

Which Royal Society president, a famous diarist, licensed the Principia for printing in 1686?

Samuel Pepys was a naval administrator, not a scientist, yet as the Society's president he put his name on the title page of Newton's great book.

Newton already had his calculus, yet the Principia's proofs mostly use which older branch of mathematics?

Geometry: the proofs argue from diagrams of vanishingly small shapes. Newton admitted the hard style was deliberate: he first wrote the final book in a popular way, then recast it as mathematical propositions to prevent disputes.

Newton's law of universal gravitation makes the pull between two bodies inversely proportional to what?

The square of the distance: double the separation and the pull drops to a quarter, treble it and only a ninth remains. It fades fast, but never quite to zero.

Who first tested Newton's gravitation between masses in a laboratory, in 1798?

Henry Cavendish sealed his lead balls in a closed room and watched the balance from outside, since the slightest draught would swamp so tiny a pull. His real aim was the density of the Earth.

Isaac Newton was born on which holiday in 1642?

Christmas Day 1642 on England's old Julian calendar, which is 4 January 1643 on today's. Born early, he was a tiny, weak baby whom nobody expected to live.

Which Cambridge institution admitted Newton in June 1661?

Trinity took him in as a sizar, a student who earned his keep by waiting on wealthier ones, even though his mother could comfortably have paid his way.

Which German mathematician shares credit with Newton for inventing calculus?

Gottfried Wilhelm Leibniz got no fair hearing in the priority row: the Royal Society's president, Newton himself, picked the committee and secretly wrote its report, which found for Newton.

Which monarch knighted Newton in 1705?

Queen Anne dubbed him at Cambridge a month before a general election in which he was standing, most likely to lift his chances. If so, it failed: Sir Isaac was soundly beaten.

Which institution did Newton run as Master from 1699 until his death?

The Royal Mint job was meant as a comfortable reward, but Newton took it seriously: he became the terror of London's counterfeiters and sent a good number of them to the gallows.

Isaac Newton shared his name with which relative, who died three months before he was born?

His father, a farmer also called Isaac, never saw his son. His mother soon remarried and moved away, leaving the small boy with his grandmother until his stepfather died in 1653.

In which book, published in 1704, did Newton collect his work on light?

Opticks went unpublished for years because of a feud: Newton had quarrelled bitterly with Robert Hooke over light, and he refused to print the book until Hooke was dead.

Newton built the first known functional example of which optical instrument?

A reflecting telescope: convinced that lenses would always smear colours, Newton used a curved mirror instead. The instrument so pleased the Royal Society that it elected him a fellow in 1672.

Where did Newton's body lie in state before his burial in 1727?

Westminster Abbey gave him a send-off fit for a statesman: the Lord Chancellor, two dukes and three earls carried the pall, and most of the Royal Society walked behind the coffin.

About which aspect of gravity did Newton famously write 'I frame no hypotheses'?

Its cause: Newton could say exactly how gravity behaves but not why, and in 1713 he refused to guess. Whatever is not drawn from observation, he wrote, has no place in experimental philosophy.

In a Newton's cradle, what happens at the far end when a raised pair of balls is released?

Two balls swing out, rising almost as high as the pair that was let go, while the ones in between hardly stir: the cradle always sends out as many balls as came in.

Which two modern theories address the cases where Newton's laws break down?

Quantum mechanics and relativity take over for things smaller than a molecule or moving near the speed of light. Everyday life sits between those extremes, which is why Newton's physics feels like common sense.

As a car pulls away its tyres shove backwards, so what shoves the car forwards in return?

The road: the engine only spins the wheels, and it is the road's equal push back on the tyres that moves the car. On sheet ice the tyres get no grip, and the car goes nowhere.

In rotational motion, what is the counterpart of force?

Torque is a force multiplied by its distance from the pivot, which is why door handles sit far from the hinges: push right beside them and the door barely moves.

Which car fitting prevents whiplash, when a rear-end crash shoves the torso forward faster than the neck can follow?

Head restraint is the formal name for a headrest, and it is safety kit, not a cushion. The seat drives the torso forward while the head, by Newton's first law, tends to stay where it was.

In which sport do teammates scrub the ice so a sliding granite rock meets less friction and travels farther?

Curling sweepers slightly melt the ice under their brooms, so the rock runs farther and straighter. With less friction to slow it, the heavy granite simply keeps going, as Newton's first law says.

Which sea animals flee predators rocket-style, blasting water one way through a funnel to shoot the other way?

Squid suck water into the body and blast it out of a funnel in a fast, strong jet, steering by aiming the funnel. Flying squid can even leave the sea and glide through the air for up to 50 metres.

Which newspaper mocked Robert Goddard in 1920 for saying rockets work in a vacuum, then printed a correction in 1969?

The New York Times ran its correction with the first Moon-landing crew on its way, admitting that a rocket can function in a vacuum as well as in an atmosphere. It ended: 'The Times regrets the error.'

Gravity is still strong aboard the International Space Station, so astronauts float because they are in what?

Free fall is the whole trick: the station and its crew drop as one, so nothing presses anyone against the floor. Going sideways fast enough, not a lack of gravity, is what keeps a satellite up.

What name is given to a rifle's kick, which pushes the shooter as hard as the gases push the bullet?

Recoil is the price of every shot: the gun is driven rearward with just the same force that drives the bullet forward. A heavier gun kicks more gently, because that shove has more mass to move.

Bill Anders joked that Isaac Newton was 'doing most of the driving' on which first crewed flight around the Moon?

Apollo 8 was coasting home when capsule communicator Michael Collins passed on his young son's question: who's driving? With the engine off, gravity and Newton's laws were doing the steering.

To which rival, famed for his microscope drawings, did Isaac Newton write of 'standing on the shoulders of giants'?

Robert Hooke received the phrase in a letter of 1675. Newton was borrowing it, too: Bernard of Chartres had compared scholars to dwarfs perched on giants about five centuries earlier.

Which NASA spacecraft was lost in 1999 after software mixed up pound-force seconds and newton-seconds?

Mars Climate Orbiter came in far too close to the planet and was lost, most likely torn apart in its atmosphere. The thruster figures were wrong by a factor of 4.45, the number of newtons in one pound of force.

Which 1977 NASA probe has coasted for decades, engine off, to become the most distant human-made object?

Voyager 1 needs no engine to keep going: with almost nothing out there to slow it, Newton's first law carries it along at about 17 kilometres every second.

Which way does coffee slop from a brim-full cup when the car carrying it pulls away sharply from rest?

Backward, into the driver's lap: coffee at rest tends to stay at rest while the car accelerates out from under it. Brake hard and the same coffee carries on forward into the dashboard.

Which French philosopher, of 'I think, therefore I am', published laws of motion decades before Newton's Principia?

René Descartes had written down the idea of inertia by 1633 but shelved that book to stay clear of controversy, and it came out only after his death. His laws of nature reached print in 1644.

Which garment squeezes a fighter pilot's legs during tight turns to stop blood pooling there and starving the brain?

Anti-g suit trousers hold bladders that inflate and press on the legs and abdomen. In a hard turn the jet changes course while the pilot's blood tends to keep its old path, draining away from the head.

What is the pulling force passed along a taut rope, chain or string called, as in a tug of war?

Tension only ever pulls: a rope can tug on whatever is tied to each end, but it goes slack the moment anyone tries to push with it.

Which 'fictitious' force, felt on a spinning planet, makes cyclones turn opposite ways north and south of the equator?

Coriolis force deflects moving air to the right north of the equator and to the left south of it. It is named after the Frenchman whose 1835 paper applied it to water wheels, not weather.

What do physicists call a force multiplied by the time for which it acts, as when a bat strikes a ball?

Impulse takes its name from the short, sharp forces of impacts and collisions, where the overall effect of a blow is far easier to measure than the force at each instant.

On a helicopter, which small part stops the body spinning the opposite way to the main blades overhead?

Tail rotor thrust pushes sideways on the tail to hold the body steady. Without it, an engine turning the main blades one way would spin the rest of the helicopter the other way.

The paired forces of Newton's third law, like foot on ball and ball on foot, never cancel because they act on what?

Different objects feel the two forces: the foot pushes the ball and the ball pushes the foot. Forces can cancel only when they act on the same thing.

Galileo pictured butterflies flitting evenly round a shut room to argue that steady motion cannot be felt aboard what?

A ship sailing smoothly was Galileo's test case: below decks, insects, dripping water and anything thrown behave exactly as they do in harbour. Nothing done inside can tell steady motion from rest.

Which name did Isaac Newton coin for the inward force that keeps planets in orbit or a whirled ball on its string?

Centripetal force means 'centre-seeking', and it is a job, not a separate kind of force: gravity does it for a planet, and the string does it for the ball.

Three supermarket trolleys get identical shoves; by Newton's second law, which one picks up speed fastest?

The empty one wins: the same force gives a smaller mass a bigger change in speed. It is why a push that launches a basketball barely budges a stalled car.

If the net push on a cart becomes twice as large, Newton's second law says its rate of gaining speed does what?

Doubles: with the load unchanged, the rate of gaining speed rises in step with the net force. Putting twice the load on the cart would do the opposite and cut that rate to half.

A skydiver stops gaining speed at about 200 km/h, when weight is matched by which growing force?

Air resistance grows with speed until it equals the pull of gravity; after that the net force is zero and the fall continues at a steady pace. Tucking in the limbs raises that limit.

What do physicists call a sketch that isolates one object and shows every force on it as an arrow?

Free-body diagram arrows show only the forces acting on the chosen object, never the ones it exerts. The object itself can shrink to a blob, since only the pushes and pulls count.

Which school-lab instrument, also called a newton meter, has a hook at one end for hanging loads?

Spring scale coils stretch in proportion to the load, which is what lets the dial be marked in even steps. With heavy use the coil can stretch for good and start to read wrong.

Which unit of pressure, one newton per square metre, honours a French thinker who experimented with barometers?

Pascal is a very small unit, so everyday air pressure at sea level comes to about 101,000 of them. Weather forecasters count in hectopascals, bundles of a hundred.

Which unit of energy is the work done when a force of one newton pushes something through one metre?

Joule is about the energy needed to lift one apple one metre, since a typical apple weighs roughly one newton. The unit was named for English physicist James Prescott Joule while he was still alive.

Weighing fails in orbit, so the space station's SLAMMD uses Newton's second law to find what about each astronaut?

Body mass is found by giving a floating crew member a known push and tracking how quickly they speed up. The more sluggishly the astronaut responds, the more of them there is.

Which kind of friction, usually the stronger, must be overcome to start a heavy crate sliding from rest?

Static friction is the grip that holds things still, and it usually beats the kinetic kind. That is why a stuck crate lurches the moment it finally gives.

A table holds up a resting book with which force, named with the mathematical word for 'perpendicular'?

Normal here means perpendicular, not ordinary: the table pushes directly outward from its surface, just hard enough to keep the book from sinking through. A flimsy table that cannot supply it breaks.

Which puzzle of Newtonian gravity, with no general exact solution, gave Liu Cixin's alien-contact novel its title?

The three-body problem begins as soon as another mass joins two that orbit each other: the paths turn chaotic and no general formula predicts them. Liu's aliens live among three suns.

In a classroom demonstration of Newton's third law, what is taped to a drinking straw to shoot along a string?

A balloon taped to a drinking straw on a taut string makes a simple rocket: air rushing out of the neck one way drives it the other way along the line.

Newton opened the Principia by defining mass as the quantity of what in a body?

Matter was Newton's starting point: its quantity, he wrote, arises from density and bulk together, and that quantity is what he meant by mass.

Because an orbiting planet tugs back on its star, astronomers find unseen worlds by watching the star do what?

Wobble is the giveaway: planet and star both circle their shared centre of mass, so the star sways toward us and away. That sway revealed 51 Pegasi b, the first planet found orbiting a Sun-like star.

Which older name for the study of nature appears in the full English title of Newton's Principia?

Natural philosophy covered what is now physics, chemistry and astronomy until the nineteenth century. The word 'scientist' was coined only in the 1830s, to replace 'natural philosopher'.

Which French marquise, the partner of Voltaire, made what became the standard French translation of Newton's Principia?

Émilie du Châtelet worked on it through a pregnancy at 42 that she feared she would not survive, and died days after her daughter was born. Her translation was first printed in 1756.

Which English poet wrote the epitaph 'God said, Let Newton be! and all was light' about nature's hidden laws?

Alexander Pope opened the couplet with 'Nature, and Nature's laws lay hid in night.' It was written for Newton's monument, but permission to carve it there was refused.

Which ancient engineer described the aeolipile, a hollow ball spun round by jets of its own escaping steam?

Hero of Alexandria described it in the first century, and it counts as the first recorded steam engine. Steam leaving the nozzles one way turns the ball the other way, the principle behind every rocket.

To argue for absolute space, Newton described water climbing the sides of which spinning vessel hung on a twisted cord?

A bucket of water hung on a twisted cord was Newton's evidence: the surface stays flat at first, then hollows as the water itself picks up the spin. Ernst Mach later challenged what that proves.

Newton's Principia credits Wren, Wallis and which Dutch inventor of the pendulum clock with the rules of collisions?

Christiaan Huygens had the correct rules for colliding bodies by 1656, though his full treatise on them was printed only after his death. He also discovered Titan, the largest moon of Saturn.

On perfectly frictionless ice, what would a moving hockey puck need in order to keep gliding at a steady pace?

No force is needed: nothing is trying to stop the puck, so it simply carries on. Polish any surface smoother and a sliding object travels farther each time, with no limit in sight.

A firefly splatters on a bus windscreen; compared with the bus's push on it, the insect's push on the bus is what?

Just as strong: the two pushes are always the same size. The firefly comes off worse only because that force, acting on its tiny mass, produces a far more violent change of speed.

Which unit of force from the old centimetre-gram-second system equals one hundred-thousandth of a newton?

Dyne takes its name from the Greek word for power. It is the force that speeds up a one-gram mass by one centimetre per second every second.

In the imperial system, which oddly named unit of mass gains 1 ft/s of speed each second under one pound of force?

Slug is a hefty unit, equal to about 32 pounds of mass, and early textbooks also called it the geepound. It lets engineers who think in pounds of force keep Newton's second law tidy.

Which classroom device of 1784 hangs two weights over a pulley to put Newton's laws of motion to the test?

Atwood machine was the invention of English mathematician George Atwood, built as a laboratory experiment on steadily quickening motion. It became a staple classroom demonstration.

In the Principia, Newton illustrated his third law with a horse pulled back as hard as it drags what on a rope?

A stone tied to a rope was Newton's example: the horse, he wrote, 'will be equally drawn back towards the stone'. He added that a finger pressing on one is pressed by it in return.

Which medieval theory, a step on the way to inertia, held that a thrower gives a projectile an inner motive power?

Impetus theory began with John Philoponus in the sixth century and lasted until the seventeenth. Jean Buridan gave the idea its name in the fourteenth.

Which kind of reference frame is defined as one in which Newton's first law holds true?

Inertial frames move steadily, neither turning nor changing speed. Judge motion from anywhere else, such as a braking bus, and loose things appear to slide about with nothing pushing them.

Which mathematician boasted that his 1788 Mécanique analytique, Newton's laws recast as pure algebra, had no diagrams?

Joseph-Louis Lagrange reduced every problem of motion to equations. He called Newton the greatest genius who ever lived, and the luckiest, since there is only one system of the world to find.

Which toy did Newton cite under his first law in the Principia, saying only the air slows its rotation?

A top appears beside projectiles and planets in Newton's text. Its parts are forever pulled aside from the paths they would take, he wrote, yet it 'does not cease its rotation' except as the air retards it.