50 Fun Facts About Newton's Laws of Motion

In which book did Isaac Newton first state his three laws of motion?

Its title means 'Mathematical Principles of Natural Philosophy'; most people just call it the Principia.

In what year was the Principia originally published?

Newton put out two more editions in his lifetime, a corrected one in 1713 and an improved one in 1726.

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

That is the principle of inertia: left alone, a body keeps doing exactly what it is already doing.

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

Newton's wording was that a body continues in its state 'unless it is compelled to change that state by forces impressed upon it'.

Newton's second law says the change of motion is proportional to what?

And it happens along the straight line in which that force acts, which is why the law is written with vectors today.

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

That is the famous F = ma; equivalently, force is the rate at which the body's momentum changes.

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

By the second law it does not accelerate, which covers both sitting still and coasting at constant velocity.

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

The catch students miss is that the action and the reaction act on different bodies, so they never simply cancel.

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

Because paired forces are equal and opposite, they cancel when added, so the total momentum of the system stays constant.

Which thought experiment links a projectile's flight with orbital motion?

Fire it fast enough and the ball falls away from a straight line at the same rate the Earth curves beneath it, so it never lands.

Linear momentum is the product of mass and what?

The symbol p comes from the Latin pellere, meaning to push or drive.

In SI units, momentum is measured in what?

A 1 kg model plane flying north at 1 m/s has a momentum of 1 kg·m/s due north.

What is the SI unit of force?

It is a named derived unit; force is usually written with the symbol F.

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

In base units that is 1 kg·m/s², and the name was formally adopted by the CGPM in 1948.

In what year did the CGPM adopt the name 'newton' for the unit of force?

The 9th General Conference on Weights and Measures did it by Resolution 7, two years after standardising the amount itself.

Because it has both magnitude and direction, force is what kind of quantity?

That is why two equal forces pulling in opposite directions add to nothing.

If a closed system feels no external forces, what happens to its total momentum?

Momentum is conserved in every inertial frame, and the idea survives into relativity and quantum mechanics in modified form.

In which Italian city was Galileo born in 1564?

His father Vincenzo was a lutenist and music theorist; the family moved to Florence when Galileo was ten.

Which scientist first introduced the term 'inertia'?

He took it from the Latin for 'idleness' and used it only for resistance to movement; Galileo and Newton gave it its modern sense.

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

Kepler assumed rest was the natural state of things, so inertia to him was a body's laziness about moving at all.

In science, weight is a quantity associated with which pull on an object?

Mass is how much matter there is; weight changes with the gravity around you, which is why astronauts float but do not lose mass.

Inertial mass measures an object's resistance to what?

Experiments have never found a difference between inertial mass and the gravitational mass that sets how hard gravity pulls.

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

It throws hot gas backwards at high speed, and the reaction pushes the rocket forwards, so it works just as well in a vacuum.

Friction is the force resisting what?

Dry friction splits into static friction between surfaces that are not yet moving and kinetic friction once they slide.

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

They stayed hidden in his notebooks until Guillaume Amontons rediscovered them in 1699.

Who rediscovered the laws of dry friction in 1699, and now has them named after him?

Coulomb developed the theory further in 1785, and Euler was the first to separate static from kinetic friction.

Which astronaut dropped a hammer and a feather together on the Moon in 1971?

With no air to slow the feather, both hit the lunar surface at the same moment, just as Galileo had argued.

In the Moon demonstration, what happened to the hammer and the feather?

Free-fall acceleration does not depend on mass; on Earth only air resistance makes feathers drift.

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

The story dates the experiment to 1589–1592; the fall would have been too quick for him to learn much from it.

Which 1638 work by Galileo did Einstein cite in calling him the 'father of modern physics'?

He wrote it under house arrest; it deals mainly with the motion of bodies and the strength of materials.

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

The comet man later used Newton's gravitation to predict that his 1682 comet would return, which it did in 1758.

In which language was the Principia written?

It runs to three volumes and was licensed for printing by Samuel Pepys, then president of the Royal Society, on 5 July 1686.

Which Royal Society president licensed the Principia for printing in 1686?

The famous diarist gave his imprimatur on 5 July 1686, a year before the book appeared.

How many further editions of the Principia did Newton publish after the first?

He corrected errors in 1713 and improved the text again in 1726, a year before he died.

Newton's law of universal gravitation says force falls with what?

Double the separation and the pull drops to a quarter; it is an inverse-square law like Coulomb's law for charges.

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

His torsion-balance experiment let later physicists put a number on the gravitational constant G.

Where was Isaac Newton born?

He arrived on Christmas Day 1642 by the old calendar, three months after his father died, and so small he could fit in a quart mug.

Which Cambridge institution admitted Newton in June 1661?

He became its second Lucasian Professor of Mathematics at 26, taking over from Isaac Barrow in 1669.

Which German mathematician shares credit with Newton for inventing calculus?

Newton had developed his version years earlier but published late, and the priority dispute turned bitter.

Which monarch knighted Newton in 1705?

It happened during a royal visit to Trinity College, Cambridge, in April that year.

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

He was Warden first from 1696 and tightened the accuracy and security of British coinage, pursuing counterfeiters personally.

Newton's father, also named Isaac, died how long before his son was born?

Born prematurely, the baby was so small his mother said he could have fitted inside a quart mug.

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

His experiments with prisms showed white light is a mixture of colours and led to the first reflecting telescope.

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

The design, now called a Newtonian, uses a curved mirror instead of a lens and so avoids colour fringing.

Where did Newton's body lie in state for eight days before burial?

He was buried in the nave in 1727, an honour usually reserved for royalty and statesmen.

Which Latin word did Newton use for the effect that became known as gravity?

It literally means weight; he famously said he 'framed no hypotheses' about what caused it.

The desk toy Newton's cradle was actually designed by which French scientist?

Its swinging balls demonstrate conservation of momentum and energy, which is why it carries Newton's name.

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

Newton's laws remain an excellent approximation for everyday speeds and sizes, which is why they are still taught first.

What is the study of the behaviour of massive bodies using Newton's laws called?

Lagrangian and Hamiltonian mechanics later reformulated the same physics in terms of energy rather than force.

In rotational motion, what is the counterpart of force?

Mass becomes moment of inertia and momentum becomes angular momentum, so the laws carry over almost word for word.