sleep a time-facts, forces, let your body sink into the mattress, feel the weight of the blanket resting
across your shoulders. The gentle pressure of your head against the pillow, that steady downward
pull is gravity, the same force that holds the moon in its slow path around the earth, and keeps
a cup of tea from floating off the nightstand, it never sleeps, and it never hurries, it simply draws
everything toward everything else, quietly, and without strain, breathe in slowly, notice how
your chest rises. A small motion against this constant attraction, as you exhale, let your
muscles soften a little more, the dark around you, is not empty, gravity threads through it,
linking your resting body to the floor, the house, the soil outside, all the way down to the
planet's core in the quiet of this night. That pull is a companion, it asks nothing, it only holds,
over the coming episodes, we will meet other forces. Each one woven into ordinary moments,
but right now, the only one you need to notice is the one cradling you where you lie, let your breathing
slow, the night is calm, gravity is gentle, sleep will come, a stone and a feather, dropped from the same height,
reach the ground at different moments, the stone falls straight and fast, the feather drifts,
hesitates, and lands softly a breath later for most of human history. This ordinary observation
was taken as proof that heavier objects fall faster, the idea feels intuitive, a cart loaded with bricks,
is harder to pull than an empty one, so surely gravity must tug more urgently on greater mass,
the problem with that reasoning, is that it mixes up two distinct things, the pull of gravity,
and the interference of air, Galileo Galilei, working in the late 16th and early 17th centuries,
thought about the problem differently, he proposed that in the absence of air, all objects would fall together,
regardless of their weight, a famous story places him, atop the leaning tower of Pisa,
dropping spheres of different masses to demonstrate the principle to a crowd, the tower leans at a gentle
angle, and from its upper galleries, you can see the piazza below whether Galileo ever performed,
that public demonstration is uncertain, but he did conduct careful experiments with inclined planes,
a ball rolling down a shallow slope, moves more slowly than one falling straight down,
which allowed him to measure motion with the tools of his time, he found that the distance travelled
grew with the square of the time, meaning the ball gained speed, steadily, the mass of the ball,
did not change that steady gain, the key insight was that gravity accelerates everything at the same rate,
the force of gravity pulling on an object is larger when the object is heavier,
but the object's resistance to being accelerated, it's inertia, is also larger by exactly the same
proportion, the two effects cancel cleanly, a bolder feels a stronger gravitational tug than a pebble,
but it is harder to move by the same factor, the outcome is a shared acceleration,
roughly 9.8 meters per second squared at the surface of the earth in a vacuum chamber on earth,
you can watch a coin and a feather tumble side by side and hit the floor at the same instant,
a quiet confirmation of Galileo's reasoning, the most vivid demonstration,
came not from a laboratory vacuum, but from the surface of another world in 1971,
during the Apollo 15 mission, Commander David Scott stood on the moon in his white suit and held
a feather and a hammer before the camera, the moon has no atmosphere to speak of,
so there was no air resistance to complicate things he let go of both objects at the same moment
they fell together, a falcon feather and a heavy geology hammer, and landed in the great dust at the
same time, the image is strangely peaceful, the objects dropped slowly, because the moon's gravity is
weaker, about a 6th of earth's, but the equality of their descent was perfect, science became something you could
see with your own eyes, a silent ballet of mass and motion Isaac Newton later gave the phenomenon
its mathematical shape he described a universal force that pulls every particle of matter toward
every other particle, with a strength that depends on the masses involved and the square of the distance
between them, the same law that guides an apple to the ground, keeps the moon sailing around the earth,
the moon is always falling toward us, but because it has enough sideways speed, it keeps missing,
the curve of its path matches the curve of the earth beneath it, the equality of acceleration
that Galileo uncovered is built into Newton's equation, the mass that determines how much force
and object experiences, also determines how much it resists changing its motion, they are the same number
sitting on both sides of the equation and they cancel in everyday life, we do not usually
see this equality, because air is everywhere around us, a leaf spiraling down from a tree is shaped
by air resistance, its large surface catches the breeze, slowing its fall until it reaches a steady
gentle speed that gentle drift is beautiful, but it hides the simple rule underneath in the stillness
of a vacuum, leaf and stone would fall as one, the same patient gravity pulls on the cat,
stretching on the window sill, on the rain drop sliding down the glass, and on the distant clouds
that sit motionless in a blue afternoon sky, none has a special status, each is given the same downward
push, and each would accelerate in exactly the same way if the air were not there to blur the story,
the leaning tower, the moon's feather, and the quiet vacuum chamber all point to one fact, falling
is not a contest between heavy and light, it is a shared experience governed by a single number,
a steady acceleration that does not care what a thing is made of or how large it looms when you
see an apple fall from a branch, you are watching something that applies equally to moons and dust
modes, the branch lets go, and gravity does the rest, the same for everyone, every time place your
fingertip lightly on a wooden table, and try to slide it forward, you will feel a gentle resistance,
before motion even begins, that quiet pushback is friction, a force that arises wherever two surfaces meet,
it is not a single large phenomenon, but a collection of tiny interactions happening at the scale
of microscopic hills and valleys, even surfaces that look polished to the naked eye, are
under magnification, rough landscapes of peaks and grooves when those landscapes press together,
the high points catch on one another, overcoming that catching takes a small amount of force,
and that is what we sense as a reluctance to slide, the earliest systematic thinking about friction
comes from Leonardo da Vinci who sketched experiments with blocks on inclined planes and noted
that the resistance depends on how hard the surfaces are pressed together, not on the apparent
area of contact, his notebooks lay forgotten for centuries, so the laws of friction were
rediscovered later by the French physicist Guillaume Amontins, in the late 1600s, Amontins stated
that the friction force is proportional to the load pressing the surfaces together and independent
of the size of the objects, it was a curious finding, a brick standing upright and the same brick
lying flat experience, the same friction when sliding on a given surface, even though the contact patch
looks very different, the explanation lies in those microscopic contacts as the load increases,
more of the tiny peaks to form and make real contact, and that real contact area
grows in proportion to the load, what we think of as a surface touching another, is mostly empty space
with just the tips of the roughness, in true connection, rubbing your hands together on a chilly morning,
illustrates a second aspect of friction, it transforms movement into warmth when surface is
slide, the microscopic junctions are repeatedly made, stretched, and broken, the work done in breaking
these bonds becomes tiny vibrations throughout the material, which we perceive as heat, this conversion
is not perfectly efficient, but it is reliable, people have used friction to start fires for thousands
of years, spinning a stick against a base board until the dust smolders in a gentler form,
the same process warms your palms, a book resting on a slightly tilted surface shows,
how friction can hold things still without it, the book would slide down at the smallest
incline, the force that opposes that downward tug is called static friction, and it adjusts itself
up to a certain limit, push the book lightly with one finger, and it stays put, push harder,
and it stays still, because the peaks and valleys at the interface are deforming
elastically, and interlocking, to resist the sheer, there is a maximum value, beyond which the book
breaks free, and begins to slide once it is sliding, the resistance drops a little, this difference
between static and kinetic friction, is why it can feel easier to keep something moving than to
start it from rest, the French engineer Charles Augustine de Coulomb, refined these ideas in the 1780s,
showing that the kinetic friction force is roughly constant for a given pair of materials,
and proportional to the normal load, Coulomb also observed that friction does not greatly depend on speed,
at least for ordinary materials over moderate ranges that is a quiet convenience,
a pencil on paper, a chair sliding across a wooden floor, the soul of a shoe against pavement,
all behave in fairly predictable ways, walking itself relies on static friction between your shoe
and the ground when you push backward with your foot, the roughness of the soul and the pavement interlock,
preventing your foot from slipping, and the reaction force propels you forward on a polished
frozen puddle, the interlocking is so reduced that walking becomes uncertain, there is a small aid,
we give friction without thinking about it, a dusty shelf can make a jar feel loose when you set it down,
because the dust particles act like tiny ball bearings, reducing the interlocking, wiping the surface clean,
or pressing down a little harder, restores the grip, even the gentle rub of a fingertip across a tabletop,
is a dialogue between the ridges of your skin and the almost invisible texture of the wood
or varnish, the light vibration you feel as your finger slows, is the sound of countless
microcontacts forming and releasing in a soft flutter, the world without friction would be unrecognizable,
but the world with friction is held in a calm, steady grip, a vase stays on its shelf,
a sleeping cat stays curled on a cushion, and a spoon rests quietly beside a plate,
the surfaces around us are not perfectly smooth, and that small imperfection is a gift that keeps
things in their place, letting us move through the day with a quiet sense of stability on a quiet wall,
a painting hangs from a length of picture wire, the wire is looped over two small hooks
and fastened at the back of the frame, it sags just a little, forming a gentle curve between the
supports, nothing moves, the picture appears to float without effort, but inside the wire, a steady
force keeps everything in place, that force is tension, a silent pull that travels along cords,
ropes, cables, and threads it acts only in one way, it draws the ends of a flexible line inward,
stretching the material ever so slightly if you were to cut the wire at any point and hold both
cut ends, you would feel them tugging away from each other, the pull you feel is the tension at that
spot, it arises, because the wire is being stretched between two anchor points, and its fibers resist
being pulled apart, the picture's weight pulls downward on the wire, where it attaches to the frame
in response, the two segments of wire angle upward toward the hooks, the upward components of the
tension, in those two segments, add together to exactly balance the weight of the painting, the shallower
the sag, the harder the wire must pull horizontally, to produce enough upward lift, that is why
picture wire is allowed to drew a little, a nearly horizontal wire would need a tremendous tension,
for even a modest frame, consider a swing, hanging at rest, in an empty park, two chains reach from a
high bar, to a flat seat, each chain is made of many metal links, the top link feels a pull from the link below,
and that link from the next, all the way down, but the tension in the chain is not the same at every height,
the uppermost link must bear the weight of the seat, and also the weight of every single link that hangs beneath it,
the link just above the seat, on the other hand, supports only the seat's weight and a short portion of chain,
so the tension in a real chain increases steadily as you go upward because each section carries the weight
of everything below, if the chain had no mass of its own, the tension would be uniform from top to bottom,
but real materials have weight, and that weight adds to the load that the upper parts must sustain
the same idea applies to long ropes when a cable lowers a heavy instrument into the ocean from a research vessel,
the uppermost meters of cable support the instrument, plus the many tons of cable that trail below,
engineers account for this by making the top of the cable thicker and stronger than the deep end
in a vertical rope of uniform thickness and mass, the tension at any cut is simply the weight of the rope segment below
that cut plus any attached load that straight forward rule gives engineers a way to predict the force
at every point tension distributes itself across every fiber of a rope in a braided line
hundreds of tiny strands lie side by side, each carrying a small fraction of the total pull
when the rope is still, the load is shared quietly among them, the fibers stretch a microscopic amount,
just enough for the materials internal bonds to balance the outward pull if one strand were weaker,
its neighbors would take up a little more of the load, the rope's flexibility is what allows
tension to align itself along the length of the cord, finding a straight path between the points
where the force is applied measuring tension, as a long gentle history, early builders tested the
strength of hemp ropes by hanging stone weights until the cord parted a silk thread thinner than a
hair could in a calm room hold a surprisingly heavy metal bead these simple tests revealed that a
cord's capacity depends on the material and its thickness modern instruments called load cells
can sense tension without breaking anything they convert the small stretch of a metal element into an
electrical signal letting engineers read the pull in a bridge cable or an elevator rope while the
structure is in use in a large suspension bridge the main cables hang between towers in a graceful
curve under the weight of the roadway which is distributed evenly along the horizontal span
that curve becomes a parabola rather than the catenary shape a free hanging rope would take the tension
in those cables is not constant near the tops of the towers where the cables bend over saddles
the pull reaches enormous values because the entire bridge hangs from that point farther out
toward the center of the span the tension is lower every vertical suspender cable transfers its little portion
of the decks weight upward into the main cable adding to the cumulative pull that must be resisted by the
anchorages at each end when you see a spider's thread shining with morning dew the strand is
under a light steady tension it connects one twig to another and the pull within it balances the weight
of the silk itself and any tiny droplets clinging to its length outwardly it appears motionless
but at every point along its length a small force draws inward tension asks so little
of our attention a length of close line supports a clean sheet a braided string suspends a glass
prism in a window and the force within them remains constant and silent it is simply the line
along which a quiet weight comes to rest asking nothing more than to hold still you take a rubber band
between your fingers and draw it apart it lengthens it narrows in the middle and a soft resistance
builds against your hands that resistance is the spring force the quiet refusal of certain materials
to leave their original shape let go and the band snaps back to a short loop giving up all the energy
you stored in it that snap back is the defining signature of elastic things the spring force lives inside a
stretched rubber band a compressed mattress spring a pulled hair tie it is the way atoms inside a solid
talk to one another when no force is acting they sit in a relaxed arrangement each atom
a comfortable distance from its neighbors held in place by the balance between attraction and repulsion
give the material a tug and those atoms shift slightly widening some gaps and narrowing others
they don't like this new arrangement the bonds between them strain and a restoring force appears
pulling them back toward that original state the harder you pull the stronger the restoring force
grows as more and more bonds stretch out of their easy positions this relationship was first measured
clearly in the 1660s by the English scientist Robert Hook he hung weights from coiled springs
and watched how far they elongated what he found was simple and orderly at a small weight
get a small stretch double the weight and the spring stretches twice as far triple it three times as far
the force pulling on the spring and the distance it moved were directly proportional
a straight line on a graph hook expressed it in latin a tensio sick vis as the extension so the
force today we call it hoax law it holds true not just for coiled metal springs but for many elastic materials
as long as you don't tug them too hard the limit matters there's a point for every springy object beyond
which the atoms don't just strain politely they slip past one another into new positions for a paperclip bent too far
the silver colored metal stays bent for a rubber band pulled past its limit it might come back
slack and baggy never quite the same length again that's the line between elastic behavior
where the object remembers its old shape and plastic behavior where it accepts a new one as long as you
stay inside the elastic region the material is a reliable storehouse for energy the energy
itself is worth noticing when you stretch a spring or an elastic band the work your hands do
doesn't disappear it becomes potential energy tucked into the stretched bonds like a battery that holds
only one kind of charge release the band and that potential energy turns into kinetic energy
the energy of motion the band flies back the spring recoils the mattress wire pushes upward
the faster the motion the more kinetic energy appears in a perfect frictionless world all the
potential energy would become kinetic and then as the object overshoots a little and compresses
turn back into potential energy in the opposite direction setting up a gentle oscillation
that never dies real rubber bands and bed springs lose some energy to warmth to the fate
internal heating as molecule slide so the bouncing eventually settles a mattress spring works
in just this gentle dampened way when you lie down your weight compresses dozens of steel coils
each coil presses back with a force proportional to how much it has been squeezed over the course of
the night as you shift position the springs give and take storing and releasing energy in small
smooth increments no single spring ever moves very far they all stay well within their elastic limits
so the mattress keeps the same shape morning after morning there's a similar quiet in a stretched
rubber band resting on a desk holding shut a rolled up newspaper or looped around a stack of envelopes
the energy sits there patiently invisible until you peel the band away a hair tie thick with elastic
threads wrapped in soft fabric spends hours stretched around a ponytail without losing its springiness
a wooden bow pulled taught before an arrow is released stores energy in the bent limbs
a trampoline fabric stretched over hundreds of steel springs deforms when someone lands
and then throws them upward again returning almost all the energy it was given in each case the spring
force does the same quiet job receive a shape change store it as energy and then offer it back
the atoms don't learn anything new they just return to where they were ready again a single leaf
detaches from a branch overhead and begins its journey toward the ground it does not plunge it does
not cut a straight line through the air instead its ways from side to side tilts lifts for moment
and then settles lower tracing a slow wandering path something is holding it back pressing upward
against its flat surface that something is the air itself air feels thin to us barely there
but it is made of countless molecules in constant motion when an object tries to fall through it
those molecules must be pushed aside they push back the faster the object moves
the more molecules it encounters each second and the greater that push becomes this push is air resistance
sometimes called drag it always acts in the direction opposite to motion
a gentle but persistent break for a leaf the effect is pronounced because a leaf has a large surface
area compared to its weight as soon as it starts to fall air resistance begins to build at the very
first instant gravity is the only force acting and the leaf accelerates downward but within a
fraction of a second the upward force from the air grows the leaf speed increases more and more slowly
until the upward force exactly matches the downward pull of gravity at that moment the two forces
balance perfectly the leaf no longer accelerates it continues to fall but at a steady rate
that will not change unless the leaf tilts or folds that steady rate is called terminal speed
terminal speed is different for every object it depends on shape size and mass a flat broad leaf
has a terminal speed low enough that you can watch it drift for long seconds a drop of water
rounder and denser falls faster a p of hail falls faster still the principle does not change
only the numbers for any object falling through a fluid whether it is air or water
there is a point where resistance balances weight and acceleration ends a human body and freefall
illustrates the same balance on a larger scale a person stepping out of an aircraft does not experience
a continuous rush of increasing speed after a few seconds air resistance grows large enough to cancel
gravity in a spread equal position with arms and legs out to present the most surface area to the
oncoming air a skydiver reaches a terminal speed of roughly 55 meters per second the sensation is not
one of falling in the stomach dropping sense but of resting on a dense cushion of moving air by changing
body shape pulling the limbs in to become more compact the skydiver can reduce surface area
raise the terminal speed and to send faster stretching out again slows the descent the whole
descent from that point onward is a quiet equilibrium a steady drift governed by the same rules
that carry the leaf a maple seed or Samara adds one more layer to the story it is shaped like a
single wing with the seed itself as a small dense weight at one end when a Samara breaks from the tree
it does not tumble or flutter it begins to spin the wing catches the air in a way that generates lift
much like the blade of a miniature helicopter though it is not powered this spinning motion is called
auto rotation as the seed whirls its wing pushes against the air and the air pushes back with a
vertical component that opposes gravity the result is a further reduction in descent speed a maple
seed can take several seconds to fall the height of a two story house and during those seconds
it may travel a surprising horizontal distance carried by the slightest breeze the shape that lets
it spin softly also spreads the seeds far from the parent tree a quiet piece of natural engineering
built entirely on air resistance and lift the understanding of air resistance began to sharpen
in the 17th century when scientists moved away from the idea that heavier things fall faster
simply because they are heavy Galileo proposed that in a vacuum where no air is present to resist
all objects regardless of weight would accelerate at the same rate and hit the ground together
that idea was later demonstrated dramatically on the moon but for centuries people could see that air
was the complicating factor a feather and a coin dropped in a tube from which the air had been pumped
fall side by side with air present the feather lags because its shape gives it a very low
terminal speed the coin with its small surface area relative to weight cuts through the air much more
efficiently scientists began to measure the drag force systematically noting that it depends on the
density of the fluid the speed of the object the cross-sectional area presents and a number
known as the drag coefficient that captures how streamlined the shape is a smooth rounded shape
slips through the air with less resistance a broad irregular shape meets more all of it is the same
underlying phenomenon a fluid resists the motion of anything moving through it and the faster the
motion the stronger the resistance up to the point of balance on a still autumn day you can stand beneath
the tree and watch this physics play out in complete silence leaves peel away one by one and each
finds its own path some tumble edge over edge some rock back and forth like cradles some spiral tightly
they all take their time the air does not merely surround them it supports them holding them in a
long soft letting go that ends only when the ground gently stops their motion imagine a block of
steel solid and heavy resting in the palm of your hand it has a certain weight a certain dense
presence if you lower that same block into a basin of water it sinks immediately straight to the
bottom the steel itself is much denser than water so gravity pulls it down with force now picture a
great ocean liner built from thousands of tons of steel carrying passengers cargo fuel and machinery
it does not plunge to the sea floor it rests calmly on the surface rising and falling with the swells
the question is simply what changed the steel itself remains the same material the water does not suddenly
become thicker or more supportive the answer lives in the shape and in a principle first understood
over two thousand years ago in the third century before the common era a mathematician named
our comedies lived in the Greek city of Syracuse one story passed down through the centuries
describes him stepping into a full bath and noticing the water spill over the sides he realized that
the volume of water that overflowed matched exactly the volume of his own body that entered the
tub this connection between a submerged object and the water it pushes aside became the foundation
for understanding why things float our comedies captured the idea in a single quiet truth
any object holy or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid
displaced by the object that upward force is buoyancy water like all fluids
presses against everything it touches the pressure comes from the weight of the water above lower down
the pressure is greater higher up a little less when a solid object rests in water
the pressure pushing up on its bottom surface turns out to be slightly stronger
than the pressure pushing down on its top surface the difference between them is the buoyant force
and it always points upward the size of that force depends only on how much water the object pushes
out of the way displaced a small amount of water and the upward push is light
displaced a large volume and the upward push grows heavier a solid steel block
displaces only its own volume which is small for its weight the weight of that displaced water
is far less than the weight of the block so gravity wins and it sinks a ship's hole by contrast
is mostly empty space the steel is rolled and welded into a thin hollow shell
that encloses a vast interior filled with air when the hull settles into the water
it pushes aside an enormous volume of liquid the weight of all that displaced water is now
much greater than it was for the solid block if the hull is shaped so that the water it displaces
weighs more than the entire ship steel air engines everything then the upward buoyant force
overtakes the downward pull of gravity the ship rises until the two forces balance
and it floats this is why a ship's designers speak of displacement tonnage the number
describes the weight of water the hull pushes aside when it sits at its designed waterline a ship
that displaces 50,000 tons of water must weigh slightly less than that when fully loaded
or it will ride too low naval architects shape every curve of the hull to spread the
displacement evenly to keep the vessel stable and level the hollow form is what makes it possible
the steel merely provides a watertight skin around the buoyant volume of air you can feel a
version of this effect with nothing more than a kitchen bowl and a sink of water and empty metal mixing
bowl placed gently on the surface floats with ease push it straight down and you sense the water
pushing back resisting turn the same bowl over so it fills with water and loses its pocket of air
and it sinks without ceremony the weight of the metal did not change only the displacement changed
a ship at sea relies on the same gentle balance it floats not because its materials are light
but because its hollow form asks the water to hold up a weight of displaced liquid that matches its
own as long as that equilibrium holds the vessel rides softly supported by the very water it pushes
aside the principle sits there quietly as it has since our comedies stepped into his bath
a calm assurance that even the heaviest things can find their place on the surface a rubber balloon
fresh from a package has a smooth, slightly powdery surface it does not seem like an object that could hold
itself against a wall without any visible support yet after a few seconds of brisk rubbing
against a wool sleeve it will do exactly that press it lightly to the wall and it stays
sometimes for hours sometimes through the night clinging with a silent steadiness the force at work
is static electricity a phenomenon that reveals how tiny imbalances in electric charge
can produce a noticeable pull between ordinary objects the balloon begins electrically neutral its rubber
surface contains roughly equal numbers of protons which carry positive charge and electrons
which carry negative charge wool like many fabrics also has a balance of charges in its
resting state but when the two materials are rubbed together the contact is not gentle enough to leave
everything in place electrons being far lighter and more mobile than the protons locked inside
atomic nuclei can transfer from one surface to the other wool has a greater tendency to lose electrons
while rubber has a greater tendency to gain them this is not a coincidence it is a predictable
property that scientists have arranged into what is called the tribal electric series materials higher
on the series tend to give up electrons and those lower down tend to capture them wool sits above rubber
so in the rubbing electrons migrate from the sleeve into the surface of the balloon the balloon
ends up with a net negative charge the sleeve is left with a net positive charge
though its effect is often less obvious because the fabric is thicker and the charge may spread
into the surrounding air or into moisture on the skin the balloon on the other hand is a thin insulating
membrane it's newly acquired electrons cannot travel far through the rubber so they remain crowded
on the surface with a rubbing occurred this is the moment when the balloon becomes capable of
clinging a wall in a typical house is often made of painted drywall wood paneling or plaster none of these
are strongly conductive but they are not perfect insulators either the atoms and molecules in the wall
contain charges that can shift slightly even if they cannot flow freely when the negatively charged
balloon is brought close to the wall its electric field pushes against the electrons in the wall surface
nudging them a tiny distance deeper into the material the wall surface is left with a slight excess
of positive charge a separation of charges called polarization the negative balloon
and the positively polarized patch of wall attract each other the attraction is not strong enough to
crush the balloon or dent the wall but it is enough to resist the gentle downward pull of gravity
on the lightweight rubber the balloon remains in place because the force between the separated charges
follows a pattern that the French physicist Charles Augustine de Coulomb described in the 1780s using a
delicate torsion balance Coulomb suspended a small charged sphere on a thin wire
and brought another charged sphere near it by measuring how much the wire twisted as the spheres
repelled or attracted he was able to show that the electrostatic force grows stronger as the amount of
charge increases and weaker as the distance between the charges increases specifically the force drops off
with the square of the distance having the gap between two charged objects quadruples
the pull between them for the balloon pressed flat against the wall the distance between the negative
charges on the rubber and the induced positive charges on the wall is tiny maybe a fraction of a
millimeter across the microscopic hills and valleys of the two surfaces that close proximity
corresponds to a force strong enough to hold for a long time what makes the balloon eventually fall
is not a sudden change in the law of attraction air is slightly conductive due to the ions that
naturally drift through it created by background radiation and other subtle processes over time
some of these ions bump into the balloon's surface and slowly carry away the excess electrons water vapor
in the air speeds this up because water molecules are polar and can latch onto charges forming
tiny conductive paths in dry indoor air the balloon may cling for many hours in humid air
the charge leaks away faster and the balloon drops sooner there is no sound when it finally releases
the bond simply weakens until the weight of the balloon overcomes it and it drifts to the floor the same
principle shows up in other quiet ways around a home a sock pulled from a dryer might stick to a
shirt for the same reason that the balloon sticks to the wall dust settling on a television screen
or a computer monitor is drawn by electrostatic attraction the tiny particles polarized by the charged
surface walking across a carpet and then touching a metal door knob can produce a small spark
which is the sudden equalization of charge between your body and the metal these events are all expressions
of the same force one that operates without motion or visible linkage simply because out of place
electrons create a field that reaches across the space between objects and pulls them gently
together in a quiet room a balloon on a wall is a demonstration of how balanced the world normally is
and how even a tiny upset in that balance a few million extra electrons on a rubber surface
can produce a force steady enough to hold a lightweight object in place silent and still
until the balance slowly restores itself if you place a bar magnet on a table and lay a sheet of paper over it
then dust the paper with fine iron filings something quiet and orderly appears the tiny dark
grains which a moment before scattered at random now shift and settle into arcs that sweep from one
end of the magnet to the other the lines are not solid threads they are a femoral formed by each
filing aligning itself with an invisible influence this pattern is a map of the magnetic field a magnetic
field is the region around a magnet or a magnetic force can be detected the field has both strength
and direction at every point the direction is defined by the way a small test magnet such as a
compass needle would align by convention the field lines run from the north pole of the magnet
to its south pole outside the magnet and they continue through the inside to form closed loops
inside the magnet they travel from south to north the lines never cross and they are most concentrated
where the force is strongest near the poles every permanent magnet has two poles north and south
bring two north poles close together and you feel a gentle but definite push apart a
north pole and a south pole on the other hand pull toward each other this push and pull is the hallmark
of magnetism like poles repel unlike poles attract the shape of the field lines reflects this
they curve from north to south bowing outward and the density of the lines corresponds
to the intensity of the field a stronger magnet produces a denser bundle of curves the most familiar
magnetic field is the one we stand inside every day the earth itself acts as though it contains a large
bar magnet tilted about 11 degrees from its axis of rotation a compass is simply a small lightweight
magnet free to rotate horizontally it's north-seeking end points toward the earth's magnetic
south pole which lies near the geographic north pole the field lines of the planet run from the southern
magnetic region to the northern magnetic region arching through the atmosphere near the equator
they are almost parallel to the ground near the poles they dive steeply inward this steady field
guided mariners long before anyone understood what caused it the understanding grew slowly
load stones naturally magnetized chunks of the mineral magnetite were known to the ancient Greeks
and Chinese by the 12th century travelers used load stone slivers floated on water as crude
compasses in 160 the English physician and scientist William Gilbert published a book that brought
precision to the subject he carved a sphere from a load stone which he called a terrella
and traced the field around it with a small compass the pattern he saw on his miniature earth
matched what compasses showed on the full scale planet Gilbert concluded correctly
that the earth itself is a giant magnet modern permanent magnets are made from alloys of iron
nickel cobalt and other elements inside these materials tiny regions called magnetic domains act
like miniature magnets in an unmagnetized piece of iron the domains point in random directions
and cancel one another out when a strong external field aligns them the whole piece
becomes a magnet the alignment can be locked in place creating a permanent magnet that holds its field
for decades the field around such a magnet is a direct result of that collective alignment a magnetic
field can be measured in units of Tesla or Gauze a typical horseshoe magnet might produce a field
of a few hundred gauze between its poles while a refrigerator magnet is somewhat weaker the lines you see
in the iron filing picture are not physical things but representations of the force that would act
on a north pole placed at any spot the filings simply rotate until they lie along those invisible curves
tracing out a shape that is always there waiting to be seen the pattern repeats faithfully around
every bar disc or horseshoe magnet whether it is holding a note to a kitchen door
or floating a compass card at sea it is a silent steady architecture present and unvering
through which the poles reach for one another across empty space tie a stone to a string
and set it swinging in a flat circle your hand feels the string tug outward
but the force on the stone itself points inward towards your hand to keep the stone in its loop
you must pull inward through the string just as firmly as the stone pulls outward on you that inward
pull is the centripetal force its name comes from latin words meaning centerseeking
because at every moment it aims straight toward the center of the circle remove the force let go of
the string and the stone to something simple it is not fly outward along a curved path instead
it moves off in a straight line tangent to the circle at the point of release the curve vanishes the
instant the inward pull stops the same arrangement appears anytime an object moves along a
curved path a car rounding a bend a planet tracing its orbit a swing at the top of its arc
in each case there is a real physical force pointing toward the center of rotation the force may be
supplied by friction by gravity by the tension in a cable or by the push of a wall
but it always aims inward what you feel when you are the passenger in that turning car is often
described as a centrifugal push pressing you into the door that feeling however
is not a force acting on you from the outside it is your own bodies inertia your body
following its preference to move in a straight line meets the door which is curving with the car
the door then pushes inward on you supplying the centripetal force that keeps you turning remove the
doors inward push let the car slide on a surface with no grip and you would continue straight ahead
not be flung outward Christian hygians the Dutch mathematician and physicist gave the first precise
account of centripetal force in the 1670s working with pendulum clocks and observing circular
motion he worked out that the inward acceleration needed to keep an object moving at a steady
speed in a circle depends on the square of its speed and the radius of the circle a faster object
needs a stronger centripetal push and a tighter circle also demands more force if you shorten the
string on that stone and twirl it at the same speed the pull on your hand grows if you ease your
grip and let the radius increase the force becomes gentler hygians discovery became a basis for Newton's
later laws of motion and gave the world a way to understand the orbits of moons and planets
and everyday example sits in a playground on a spinning round about each rider is kept in a circular
path by the seat or the metal bar that pushes inward if a rider shifts closer to the center
the feeling of that pushed diminishes the radius is smaller so even at the same rotation
rate the required inward force lessons move outward to the rim and the force grows
because the circle is wider and the same angular speed means a higher linear speed for a car driving
around a curve the centripetal force comes from the friction between the tires and the road the tires
grip sideways and that sideward grip points toward the center of the turn on a dry textured road
surface the friction supplies this inward push reliably if you steer more sharply the radius tightens
and the needed force rises the tires adjust their bite accordingly if you slow down
the force needed drops and the turn feels gentle engineers often bank curved roads
so that a portion of the cars wait also contributes to the inward push
reducing the reliance on friction alone the slight tilt of the pavement is a quiet way to help vehicles
follow a curved path without extra effort from the driver perhaps the most serene example
of centripetal force is the orbit of a moon around its planet gravity itself becomes the inward pull
arching a straight line path into an ellipse there is no string no friction no solid push
only the mutual gravitational attraction the moon is constantly falling toward earth
in a sense but its sideways motion is fast enough that it keeps missing forever tracing a circle
the centripetal force here is gravity and its magnitude adjusts with distance and speed
just as hydrants rule describes in the kitchen a salad spinner works by the same principle
the bowl is spun and the inner wall of the basket pushes inward on the lettuce leaves
keeping them moving in a circle the water droplets however are not held as firmly
they are small and slip through the holes continuing in straight lines once they are no longer contained
the same idea scaled up operates in a centrifuge that separates liquids of different density
though in that quiet context it does its work without drama so the force that keeps an object spinning
is always inward always at right angles to the motion always the reason the object does not stray
from its curve it is present in the slow arc of a throne ball at its peak in the hum of a bicycle wheel
in the steady turning of the stars across the night sky it is a quiet dependable requirement
one that never ceases until the path becomes straight again the circle holds only as long as
something reaches in toward its center inside the quiet heart of every atom far from the gentle
wanderings of its electrons lies a gathering of particles so small that it would take 100,000 of them
placed side by side to span the width of a single atom this is the nucleus
a knot of protons and neutrons held together by a force that has no equal in the every day
world of pushes and poles the challenge is immediate and obvious
protons carry a positive electric charge and like charges repel each other with a strength
that grows rapidly as they draw near inside the cramped quarters of a carbon nucleus
six protons press against one another with an electrical repulsion that would if left alone
fling them apart in an instant yet the nucleus remains serenely intact the reason is a form of
binding that has been called with a quiet aptness the glue at the heart of matter the strong nuclear
force operates on a scale that defies ordinary intuition it reaches only across distances of about
a quadrillionth of a meter roughly the width of a proton or neutron itself beyond that it vanishes
so completely that an electron circling the nucleus feels nothing of it at all within that tiny sphere
however it acts with an intensity that swamps the electrical repulsion many times over protons and
neutrons cling to one another as if locked in a close embrace each nucleon affecting only
its immediate neighbors this short-range quality shapes every atomic nucleus in nature it is why
a helium nucleus with its two protons is stable and why a uranium nucleus packing many more protons
heaters at the edge of stability after billions of years the story of how this force was uncovered
began with a simple puzzle early in the 20th century Ernest Rutherford fired alpha particles
at a thin gold foil and observed that a tiny fraction bounced back sharply that could only happen
if almost all the atoms mass was concentrated in a miniscule central core the nucleus was discovered
but its composition raised a new question once the neutron was identified in 1932
physicists could see that the nucleus contained protons and neutrons in tight proximity
yet the protons did not fly apart to explain this the Japanese theorist Hideki Yukawa proposed
in 1935 that nucleons exchange a new kind of particle one that would carry the strong force between them
he predicted its mass and it does in years later the pie mason or pie on was found in cosmic
ray experiments confirming the basic idea today the picture reaches deeper still we understand that
protons and neutrons are themselves built from smaller entities called quarks and it is between these quarks
that the true strong force operates the carrier of that force the glue on
binds quarks with an ever-tightening grip so that isolating one is impossible what holds the nucleus
together is a residual echo of that deeper binding a kind of nuclear van der vals force
leaking from the insides of protons and neutrons the strength is still enormous at close range
removing a single nucleon from a medium-sized nucleus can require millions of electron volts of energy
far more than the chemical bonds that link atoms into molecules because the force cares nothing for
electric charge neutrons serve as a kind of extra adhesive they contribute to the strong attraction
without adding to the proton repulsion which is why larger nuclei contain an increasing proportion
of neutrons the intricate balance between the strong forces pull and the electric repulsions
push sets the pattern of stable isotopes that fill our world the iron in a fleeting
breath's oxygen the calcium in a bone the carbon in every living cell all oh their steadfastness
to this hidden arrangement if the atom could be enlarged to the size of a large sports stadium
the nucleus would appear as a small marble at its center still dense beyond belief
a pinhead containing virtually all the atoms mass and within that marble the strong force works silently
binding the pieces with an unseen hand it is a force that never reaches the skin
never tugs a falling leaf or guides a compass needle yet without it there would be no
complex atoms no chemistry no world of solid things the quiet permanence of matter night after night
and day after day rests on this smallest of all glues the room is dark and still the blanket
rests lightly over you held down by gravity which also keeps the bed frame pressed into the carpet
the wooden slats beneath the mattress push upward with a spring force just strong enough to match your
weight in the quiet a tiny dust mode drifts through a slant of moonlight it's fall slowed to a steady
creep by air resistance every object within reach sits inside a web of these gentle
balancing poles and pushes the lamp on the nightstand is motionless because friction between its base
and the wood resists any sideways slide the water in a glass on the shelf stays level
under gravity's steady downward tug your own body warm and heavy is held in place by tension
in the sheets and the buoyant give of the pillow surrendering to sleep feels very much like this
a slow release into the net of forces that have been quietly cradling you all along