WEBVTT

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sleep a time-facts, forces, let your body sink into the mattress, feel the weight of the blanket resting

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across your shoulders. The gentle pressure of your head against the pillow, that steady downward

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pull is gravity, the same force that holds the moon in its slow path around the earth, and keeps

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a cup of tea from floating off the nightstand, it never sleeps, and it never hurries, it simply draws

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everything toward everything else, quietly, and without strain, breathe in slowly, notice how

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your chest rises. A small motion against this constant attraction, as you exhale, let your

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muscles soften a little more, the dark around you, is not empty, gravity threads through it,

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linking your resting body to the floor, the house, the soil outside, all the way down to the

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planet's core in the quiet of this night. That pull is a companion, it asks nothing, it only holds,

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over the coming episodes, we will meet other forces. Each one woven into ordinary moments,

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but right now, the only one you need to notice is the one cradling you where you lie, let your breathing

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slow, the night is calm, gravity is gentle, sleep will come, a stone and a feather, dropped from the same height,

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reach the ground at different moments, the stone falls straight and fast, the feather drifts,

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hesitates, and lands softly a breath later for most of human history. This ordinary observation

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was taken as proof that heavier objects fall faster, the idea feels intuitive, a cart loaded with bricks,

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is harder to pull than an empty one, so surely gravity must tug more urgently on greater mass,

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the problem with that reasoning, is that it mixes up two distinct things, the pull of gravity,

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and the interference of air, Galileo Galilei, working in the late 16th and early 17th centuries,

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thought about the problem differently, he proposed that in the absence of air, all objects would fall together,

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regardless of their weight, a famous story places him, atop the leaning tower of Pisa,

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dropping spheres of different masses to demonstrate the principle to a crowd, the tower leans at a gentle

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angle, and from its upper galleries, you can see the piazza below whether Galileo ever performed,

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that public demonstration is uncertain, but he did conduct careful experiments with inclined planes,

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a ball rolling down a shallow slope, moves more slowly than one falling straight down,

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which allowed him to measure motion with the tools of his time, he found that the distance travelled

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grew with the square of the time, meaning the ball gained speed, steadily, the mass of the ball,

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did not change that steady gain, the key insight was that gravity accelerates everything at the same rate,

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the force of gravity pulling on an object is larger when the object is heavier,

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but the object's resistance to being accelerated, it's inertia, is also larger by exactly the same

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proportion, the two effects cancel cleanly, a bolder feels a stronger gravitational tug than a pebble,

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but it is harder to move by the same factor, the outcome is a shared acceleration,

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roughly 9.8 meters per second squared at the surface of the earth in a vacuum chamber on earth,

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you can watch a coin and a feather tumble side by side and hit the floor at the same instant,

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a quiet confirmation of Galileo's reasoning, the most vivid demonstration,

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came not from a laboratory vacuum, but from the surface of another world in 1971,

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during the Apollo 15 mission, Commander David Scott stood on the moon in his white suit and held

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a feather and a hammer before the camera, the moon has no atmosphere to speak of,

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so there was no air resistance to complicate things he let go of both objects at the same moment

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they fell together, a falcon feather and a heavy geology hammer, and landed in the great dust at the

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same time, the image is strangely peaceful, the objects dropped slowly, because the moon's gravity is

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weaker, about a 6th of earth's, but the equality of their descent was perfect, science became something you could

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see with your own eyes, a silent ballet of mass and motion Isaac Newton later gave the phenomenon

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its mathematical shape he described a universal force that pulls every particle of matter toward

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every other particle, with a strength that depends on the masses involved and the square of the distance

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between them, the same law that guides an apple to the ground, keeps the moon sailing around the earth,

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the moon is always falling toward us, but because it has enough sideways speed, it keeps missing,

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the curve of its path matches the curve of the earth beneath it, the equality of acceleration

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that Galileo uncovered is built into Newton's equation, the mass that determines how much force

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and object experiences, also determines how much it resists changing its motion, they are the same number

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sitting on both sides of the equation and they cancel in everyday life, we do not usually

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see this equality, because air is everywhere around us, a leaf spiraling down from a tree is shaped

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by air resistance, its large surface catches the breeze, slowing its fall until it reaches a steady

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gentle speed that gentle drift is beautiful, but it hides the simple rule underneath in the stillness

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of a vacuum, leaf and stone would fall as one, the same patient gravity pulls on the cat,

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stretching on the window sill, on the rain drop sliding down the glass, and on the distant clouds

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that sit motionless in a blue afternoon sky, none has a special status, each is given the same downward

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push, and each would accelerate in exactly the same way if the air were not there to blur the story,

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the leaning tower, the moon's feather, and the quiet vacuum chamber all point to one fact, falling

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is not a contest between heavy and light, it is a shared experience governed by a single number,

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a steady acceleration that does not care what a thing is made of or how large it looms when you

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see an apple fall from a branch, you are watching something that applies equally to moons and dust

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modes, the branch lets go, and gravity does the rest, the same for everyone, every time place your

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fingertip lightly on a wooden table, and try to slide it forward, you will feel a gentle resistance,

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before motion even begins, that quiet pushback is friction, a force that arises wherever two surfaces meet,

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it is not a single large phenomenon, but a collection of tiny interactions happening at the scale

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of microscopic hills and valleys, even surfaces that look polished to the naked eye, are

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under magnification, rough landscapes of peaks and grooves when those landscapes press together,

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the high points catch on one another, overcoming that catching takes a small amount of force,

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and that is what we sense as a reluctance to slide, the earliest systematic thinking about friction

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comes from Leonardo da Vinci who sketched experiments with blocks on inclined planes and noted

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that the resistance depends on how hard the surfaces are pressed together, not on the apparent

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area of contact, his notebooks lay forgotten for centuries, so the laws of friction were

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rediscovered later by the French physicist Guillaume Amontins, in the late 1600s, Amontins stated

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that the friction force is proportional to the load pressing the surfaces together and independent

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of the size of the objects, it was a curious finding, a brick standing upright and the same brick

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lying flat experience, the same friction when sliding on a given surface, even though the contact patch

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looks very different, the explanation lies in those microscopic contacts as the load increases,

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more of the tiny peaks to form and make real contact, and that real contact area

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grows in proportion to the load, what we think of as a surface touching another, is mostly empty space

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with just the tips of the roughness, in true connection, rubbing your hands together on a chilly morning,

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illustrates a second aspect of friction, it transforms movement into warmth when surface is

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slide, the microscopic junctions are repeatedly made, stretched, and broken, the work done in breaking

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these bonds becomes tiny vibrations throughout the material, which we perceive as heat, this conversion

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is not perfectly efficient, but it is reliable, people have used friction to start fires for thousands

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of years, spinning a stick against a base board until the dust smolders in a gentler form,

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the same process warms your palms, a book resting on a slightly tilted surface shows,

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how friction can hold things still without it, the book would slide down at the smallest

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incline, the force that opposes that downward tug is called static friction, and it adjusts itself

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up to a certain limit, push the book lightly with one finger, and it stays put, push harder,

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and it stays still, because the peaks and valleys at the interface are deforming

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elastically, and interlocking, to resist the sheer, there is a maximum value, beyond which the book

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breaks free, and begins to slide once it is sliding, the resistance drops a little, this difference

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between static and kinetic friction, is why it can feel easier to keep something moving than to

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start it from rest, the French engineer Charles Augustine de Coulomb, refined these ideas in the 1780s,

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showing that the kinetic friction force is roughly constant for a given pair of materials,

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and proportional to the normal load, Coulomb also observed that friction does not greatly depend on speed,

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at least for ordinary materials over moderate ranges that is a quiet convenience,

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a pencil on paper, a chair sliding across a wooden floor, the soul of a shoe against pavement,

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all behave in fairly predictable ways, walking itself relies on static friction between your shoe

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and the ground when you push backward with your foot, the roughness of the soul and the pavement interlock,

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preventing your foot from slipping, and the reaction force propels you forward on a polished

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frozen puddle, the interlocking is so reduced that walking becomes uncertain, there is a small aid,

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we give friction without thinking about it, a dusty shelf can make a jar feel loose when you set it down,

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because the dust particles act like tiny ball bearings, reducing the interlocking, wiping the surface clean,

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or pressing down a little harder, restores the grip, even the gentle rub of a fingertip across a tabletop,

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is a dialogue between the ridges of your skin and the almost invisible texture of the wood

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or varnish, the light vibration you feel as your finger slows, is the sound of countless

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microcontacts forming and releasing in a soft flutter, the world without friction would be unrecognizable,

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but the world with friction is held in a calm, steady grip, a vase stays on its shelf,

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a sleeping cat stays curled on a cushion, and a spoon rests quietly beside a plate,

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the surfaces around us are not perfectly smooth, and that small imperfection is a gift that keeps

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things in their place, letting us move through the day with a quiet sense of stability on a quiet wall,

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a painting hangs from a length of picture wire, the wire is looped over two small hooks

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and fastened at the back of the frame, it sags just a little, forming a gentle curve between the

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supports, nothing moves, the picture appears to float without effort, but inside the wire, a steady

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force keeps everything in place, that force is tension, a silent pull that travels along cords,

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ropes, cables, and threads it acts only in one way, it draws the ends of a flexible line inward,

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stretching the material ever so slightly if you were to cut the wire at any point and hold both

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cut ends, you would feel them tugging away from each other, the pull you feel is the tension at that

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spot, it arises, because the wire is being stretched between two anchor points, and its fibers resist

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being pulled apart, the picture's weight pulls downward on the wire, where it attaches to the frame

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in response, the two segments of wire angle upward toward the hooks, the upward components of the

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tension, in those two segments, add together to exactly balance the weight of the painting, the shallower

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the sag, the harder the wire must pull horizontally, to produce enough upward lift, that is why

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picture wire is allowed to drew a little, a nearly horizontal wire would need a tremendous tension,

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for even a modest frame, consider a swing, hanging at rest, in an empty park, two chains reach from a

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high bar, to a flat seat, each chain is made of many metal links, the top link feels a pull from the link below,

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and that link from the next, all the way down, but the tension in the chain is not the same at every height,

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the uppermost link must bear the weight of the seat, and also the weight of every single link that hangs beneath it,

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the link just above the seat, on the other hand, supports only the seat's weight and a short portion of chain,

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so the tension in a real chain increases steadily as you go upward because each section carries the weight

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of everything below, if the chain had no mass of its own, the tension would be uniform from top to bottom,

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but real materials have weight, and that weight adds to the load that the upper parts must sustain

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the same idea applies to long ropes when a cable lowers a heavy instrument into the ocean from a research vessel,

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the uppermost meters of cable support the instrument, plus the many tons of cable that trail below,

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engineers account for this by making the top of the cable thicker and stronger than the deep end

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in a vertical rope of uniform thickness and mass, the tension at any cut is simply the weight of the rope segment below

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that cut plus any attached load that straight forward rule gives engineers a way to predict the force

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at every point tension distributes itself across every fiber of a rope in a braided line

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hundreds of tiny strands lie side by side, each carrying a small fraction of the total pull

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when the rope is still, the load is shared quietly among them, the fibers stretch a microscopic amount,

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just enough for the materials internal bonds to balance the outward pull if one strand were weaker,

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its neighbors would take up a little more of the load, the rope's flexibility is what allows

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tension to align itself along the length of the cord, finding a straight path between the points

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where the force is applied measuring tension, as a long gentle history, early builders tested the

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strength of hemp ropes by hanging stone weights until the cord parted a silk thread thinner than a

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hair could in a calm room hold a surprisingly heavy metal bead these simple tests revealed that a

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cord's capacity depends on the material and its thickness modern instruments called load cells

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can sense tension without breaking anything they convert the small stretch of a metal element into an

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electrical signal letting engineers read the pull in a bridge cable or an elevator rope while the

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structure is in use in a large suspension bridge the main cables hang between towers in a graceful

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curve under the weight of the roadway which is distributed evenly along the horizontal span

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that curve becomes a parabola rather than the catenary shape a free hanging rope would take the tension

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in those cables is not constant near the tops of the towers where the cables bend over saddles

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the pull reaches enormous values because the entire bridge hangs from that point farther out

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toward the center of the span the tension is lower every vertical suspender cable transfers its little portion

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of the decks weight upward into the main cable adding to the cumulative pull that must be resisted by the

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anchorages at each end when you see a spider's thread shining with morning dew the strand is

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under a light steady tension it connects one twig to another and the pull within it balances the weight

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of the silk itself and any tiny droplets clinging to its length outwardly it appears motionless

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but at every point along its length a small force draws inward tension asks so little

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of our attention a length of close line supports a clean sheet a braided string suspends a glass

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prism in a window and the force within them remains constant and silent it is simply the line

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along which a quiet weight comes to rest asking nothing more than to hold still you take a rubber band

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between your fingers and draw it apart it lengthens it narrows in the middle and a soft resistance

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builds against your hands that resistance is the spring force the quiet refusal of certain materials

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to leave their original shape let go and the band snaps back to a short loop giving up all the energy

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you stored in it that snap back is the defining signature of elastic things the spring force lives inside a

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stretched rubber band a compressed mattress spring a pulled hair tie it is the way atoms inside a solid

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talk to one another when no force is acting they sit in a relaxed arrangement each atom

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a comfortable distance from its neighbors held in place by the balance between attraction and repulsion

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give the material a tug and those atoms shift slightly widening some gaps and narrowing others

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they don't like this new arrangement the bonds between them strain and a restoring force appears

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pulling them back toward that original state the harder you pull the stronger the restoring force

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grows as more and more bonds stretch out of their easy positions this relationship was first measured

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clearly in the 1660s by the English scientist Robert Hook he hung weights from coiled springs

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and watched how far they elongated what he found was simple and orderly at a small weight

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get a small stretch double the weight and the spring stretches twice as far triple it three times as far

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the force pulling on the spring and the distance it moved were directly proportional

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a straight line on a graph hook expressed it in latin a tensio sick vis as the extension so the

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force today we call it hoax law it holds true not just for coiled metal springs but for many elastic materials

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as long as you don't tug them too hard the limit matters there's a point for every springy object beyond

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which the atoms don't just strain politely they slip past one another into new positions for a paperclip bent too far

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the silver colored metal stays bent for a rubber band pulled past its limit it might come back

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slack and baggy never quite the same length again that's the line between elastic behavior

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where the object remembers its old shape and plastic behavior where it accepts a new one as long as you

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stay inside the elastic region the material is a reliable storehouse for energy the energy

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itself is worth noticing when you stretch a spring or an elastic band the work your hands do

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doesn't disappear it becomes potential energy tucked into the stretched bonds like a battery that holds

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only one kind of charge release the band and that potential energy turns into kinetic energy

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the energy of motion the band flies back the spring recoils the mattress wire pushes upward

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the faster the motion the more kinetic energy appears in a perfect frictionless world all the

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potential energy would become kinetic and then as the object overshoots a little and compresses

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turn back into potential energy in the opposite direction setting up a gentle oscillation

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that never dies real rubber bands and bed springs lose some energy to warmth to the fate

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internal heating as molecule slide so the bouncing eventually settles a mattress spring works

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in just this gentle dampened way when you lie down your weight compresses dozens of steel coils

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each coil presses back with a force proportional to how much it has been squeezed over the course of

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the night as you shift position the springs give and take storing and releasing energy in small

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smooth increments no single spring ever moves very far they all stay well within their elastic limits

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so the mattress keeps the same shape morning after morning there's a similar quiet in a stretched

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rubber band resting on a desk holding shut a rolled up newspaper or looped around a stack of envelopes

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the energy sits there patiently invisible until you peel the band away a hair tie thick with elastic

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threads wrapped in soft fabric spends hours stretched around a ponytail without losing its springiness

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a wooden bow pulled taught before an arrow is released stores energy in the bent limbs

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a trampoline fabric stretched over hundreds of steel springs deforms when someone lands

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and then throws them upward again returning almost all the energy it was given in each case the spring

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force does the same quiet job receive a shape change store it as energy and then offer it back

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the atoms don't learn anything new they just return to where they were ready again a single leaf

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detaches from a branch overhead and begins its journey toward the ground it does not plunge it does

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not cut a straight line through the air instead its ways from side to side tilts lifts for moment

00:31:27.040 --> 00:31:36.240
and then settles lower tracing a slow wandering path something is holding it back pressing upward

00:31:36.240 --> 00:31:44.000
against its flat surface that something is the air itself air feels thin to us barely there

00:31:44.000 --> 00:31:51.840
but it is made of countless molecules in constant motion when an object tries to fall through it

00:31:52.800 --> 00:31:59.280
those molecules must be pushed aside they push back the faster the object moves

00:32:00.240 --> 00:32:08.480
the more molecules it encounters each second and the greater that push becomes this push is air resistance

00:32:09.440 --> 00:32:15.280
sometimes called drag it always acts in the direction opposite to motion

00:32:16.480 --> 00:32:25.840
a gentle but persistent break for a leaf the effect is pronounced because a leaf has a large surface

00:32:25.840 --> 00:32:34.160
area compared to its weight as soon as it starts to fall air resistance begins to build at the very

00:32:34.160 --> 00:32:43.040
first instant gravity is the only force acting and the leaf accelerates downward but within a

00:32:43.040 --> 00:32:51.760
fraction of a second the upward force from the air grows the leaf speed increases more and more slowly

00:32:53.040 --> 00:33:01.280
until the upward force exactly matches the downward pull of gravity at that moment the two forces

00:33:01.280 --> 00:33:09.200
balance perfectly the leaf no longer accelerates it continues to fall but at a steady rate

00:33:09.840 --> 00:33:17.760
that will not change unless the leaf tilts or folds that steady rate is called terminal speed

00:33:17.760 --> 00:33:27.200
terminal speed is different for every object it depends on shape size and mass a flat broad leaf

00:33:27.920 --> 00:33:35.600
has a terminal speed low enough that you can watch it drift for long seconds a drop of water

00:33:35.600 --> 00:33:43.600
rounder and denser falls faster a p of hail falls faster still the principle does not change

00:33:44.400 --> 00:33:51.040
only the numbers for any object falling through a fluid whether it is air or water

00:33:51.040 --> 00:34:00.480
there is a point where resistance balances weight and acceleration ends a human body and freefall

00:34:01.360 --> 00:34:10.080
illustrates the same balance on a larger scale a person stepping out of an aircraft does not experience

00:34:10.080 --> 00:34:19.600
a continuous rush of increasing speed after a few seconds air resistance grows large enough to cancel

00:34:19.600 --> 00:34:28.080
gravity in a spread equal position with arms and legs out to present the most surface area to the

00:34:28.080 --> 00:34:39.760
oncoming air a skydiver reaches a terminal speed of roughly 55 meters per second the sensation is not

00:34:39.760 --> 00:34:48.800
one of falling in the stomach dropping sense but of resting on a dense cushion of moving air by changing

00:34:48.800 --> 00:34:57.120
body shape pulling the limbs in to become more compact the skydiver can reduce surface area

00:34:58.240 --> 00:35:06.800
raise the terminal speed and to send faster stretching out again slows the descent the whole

00:35:06.800 --> 00:35:16.400
descent from that point onward is a quiet equilibrium a steady drift governed by the same rules

00:35:16.400 --> 00:35:24.880
that carry the leaf a maple seed or Samara adds one more layer to the story it is shaped like a

00:35:24.880 --> 00:35:34.080
single wing with the seed itself as a small dense weight at one end when a Samara breaks from the tree

00:35:35.280 --> 00:35:44.320
it does not tumble or flutter it begins to spin the wing catches the air in a way that generates lift

00:35:44.320 --> 00:35:54.240
much like the blade of a miniature helicopter though it is not powered this spinning motion is called

00:35:54.240 --> 00:36:03.760
auto rotation as the seed whirls its wing pushes against the air and the air pushes back with a

00:36:03.760 --> 00:36:12.480
vertical component that opposes gravity the result is a further reduction in descent speed a maple

00:36:12.560 --> 00:36:21.040
seed can take several seconds to fall the height of a two story house and during those seconds

00:36:21.920 --> 00:36:30.880
it may travel a surprising horizontal distance carried by the slightest breeze the shape that lets

00:36:30.880 --> 00:36:40.240
it spin softly also spreads the seeds far from the parent tree a quiet piece of natural engineering

00:36:41.120 --> 00:36:50.320
built entirely on air resistance and lift the understanding of air resistance began to sharpen

00:36:50.960 --> 00:37:00.080
in the 17th century when scientists moved away from the idea that heavier things fall faster

00:37:00.080 --> 00:37:08.720
simply because they are heavy Galileo proposed that in a vacuum where no air is present to resist

00:37:09.680 --> 00:37:18.160
all objects regardless of weight would accelerate at the same rate and hit the ground together

00:37:18.160 --> 00:37:27.680
that idea was later demonstrated dramatically on the moon but for centuries people could see that air

00:37:28.320 --> 00:37:36.480
was the complicating factor a feather and a coin dropped in a tube from which the air had been pumped

00:37:36.480 --> 00:37:44.800
fall side by side with air present the feather lags because its shape gives it a very low

00:37:44.800 --> 00:37:54.080
terminal speed the coin with its small surface area relative to weight cuts through the air much more

00:37:54.080 --> 00:38:02.320
efficiently scientists began to measure the drag force systematically noting that it depends on the

00:38:02.320 --> 00:38:11.120
density of the fluid the speed of the object the cross-sectional area presents and a number

00:38:11.120 --> 00:38:20.320
known as the drag coefficient that captures how streamlined the shape is a smooth rounded shape

00:38:20.320 --> 00:38:30.160
slips through the air with less resistance a broad irregular shape meets more all of it is the same

00:38:30.240 --> 00:38:39.360
underlying phenomenon a fluid resists the motion of anything moving through it and the faster the

00:38:39.360 --> 00:38:49.280
motion the stronger the resistance up to the point of balance on a still autumn day you can stand beneath

00:38:49.280 --> 00:38:59.280
the tree and watch this physics play out in complete silence leaves peel away one by one and each

00:38:59.280 --> 00:39:09.280
finds its own path some tumble edge over edge some rock back and forth like cradles some spiral tightly

00:39:09.280 --> 00:39:17.600
they all take their time the air does not merely surround them it supports them holding them in a

00:39:17.600 --> 00:39:27.040
long soft letting go that ends only when the ground gently stops their motion imagine a block of

00:39:27.040 --> 00:39:35.920
steel solid and heavy resting in the palm of your hand it has a certain weight a certain dense

00:39:35.920 --> 00:39:44.720
presence if you lower that same block into a basin of water it sinks immediately straight to the

00:39:44.720 --> 00:39:54.240
bottom the steel itself is much denser than water so gravity pulls it down with force now picture a

00:39:54.240 --> 00:40:04.960
great ocean liner built from thousands of tons of steel carrying passengers cargo fuel and machinery

00:40:04.960 --> 00:40:13.440
it does not plunge to the sea floor it rests calmly on the surface rising and falling with the swells

00:40:13.520 --> 00:40:23.040
the question is simply what changed the steel itself remains the same material the water does not suddenly

00:40:23.040 --> 00:40:32.560
become thicker or more supportive the answer lives in the shape and in a principle first understood

00:40:32.560 --> 00:40:40.640
over two thousand years ago in the third century before the common era a mathematician named

00:40:40.640 --> 00:40:48.880
our comedies lived in the Greek city of Syracuse one story passed down through the centuries

00:40:49.920 --> 00:40:59.120
describes him stepping into a full bath and noticing the water spill over the sides he realized that

00:40:59.120 --> 00:41:07.360
the volume of water that overflowed matched exactly the volume of his own body that entered the

00:41:07.360 --> 00:41:16.640
tub this connection between a submerged object and the water it pushes aside became the foundation

00:41:16.640 --> 00:41:24.560
for understanding why things float our comedies captured the idea in a single quiet truth

00:41:26.000 --> 00:41:36.160
any object holy or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid

00:41:36.800 --> 00:41:43.520
displaced by the object that upward force is buoyancy water like all fluids

00:41:44.480 --> 00:41:52.160
presses against everything it touches the pressure comes from the weight of the water above lower down

00:41:52.160 --> 00:41:58.400
the pressure is greater higher up a little less when a solid object rests in water

00:41:59.360 --> 00:42:05.360
the pressure pushing up on its bottom surface turns out to be slightly stronger

00:42:06.160 --> 00:42:13.680
than the pressure pushing down on its top surface the difference between them is the buoyant force

00:42:14.800 --> 00:42:23.760
and it always points upward the size of that force depends only on how much water the object pushes

00:42:23.760 --> 00:42:30.480
out of the way displaced a small amount of water and the upward push is light

00:42:30.480 --> 00:42:37.760
displaced a large volume and the upward push grows heavier a solid steel block

00:42:38.160 --> 00:42:45.920
displaces only its own volume which is small for its weight the weight of that displaced water

00:42:46.720 --> 00:42:55.440
is far less than the weight of the block so gravity wins and it sinks a ship's hole by contrast

00:42:56.240 --> 00:43:03.440
is mostly empty space the steel is rolled and welded into a thin hollow shell

00:43:04.640 --> 00:43:10.800
that encloses a vast interior filled with air when the hull settles into the water

00:43:10.800 --> 00:43:20.160
it pushes aside an enormous volume of liquid the weight of all that displaced water is now

00:43:20.160 --> 00:43:28.480
much greater than it was for the solid block if the hull is shaped so that the water it displaces

00:43:28.480 --> 00:43:39.200
weighs more than the entire ship steel air engines everything then the upward buoyant force

00:43:39.920 --> 00:43:46.560
overtakes the downward pull of gravity the ship rises until the two forces balance

00:43:47.440 --> 00:43:54.160
and it floats this is why a ship's designers speak of displacement tonnage the number

00:43:54.160 --> 00:44:03.440
describes the weight of water the hull pushes aside when it sits at its designed waterline a ship

00:44:03.440 --> 00:44:11.520
that displaces 50,000 tons of water must weigh slightly less than that when fully loaded

00:44:12.800 --> 00:44:19.920
or it will ride too low naval architects shape every curve of the hull to spread the

00:44:19.920 --> 00:44:28.800
displacement evenly to keep the vessel stable and level the hollow form is what makes it possible

00:44:28.800 --> 00:44:37.520
the steel merely provides a watertight skin around the buoyant volume of air you can feel a

00:44:37.520 --> 00:44:46.000
version of this effect with nothing more than a kitchen bowl and a sink of water and empty metal mixing

00:44:46.000 --> 00:44:54.880
bowl placed gently on the surface floats with ease push it straight down and you sense the water

00:44:54.880 --> 00:45:04.000
pushing back resisting turn the same bowl over so it fills with water and loses its pocket of air

00:45:05.200 --> 00:45:13.120
and it sinks without ceremony the weight of the metal did not change only the displacement changed

00:45:13.120 --> 00:45:20.800
a ship at sea relies on the same gentle balance it floats not because its materials are light

00:45:20.800 --> 00:45:31.360
but because its hollow form asks the water to hold up a weight of displaced liquid that matches its

00:45:31.360 --> 00:45:40.720
own as long as that equilibrium holds the vessel rides softly supported by the very water it pushes

00:45:40.720 --> 00:45:47.920
aside the principle sits there quietly as it has since our comedies stepped into his bath

00:45:49.280 --> 00:45:57.680
a calm assurance that even the heaviest things can find their place on the surface a rubber balloon

00:45:57.680 --> 00:46:06.800
fresh from a package has a smooth, slightly powdery surface it does not seem like an object that could hold

00:46:06.800 --> 00:46:15.120
itself against a wall without any visible support yet after a few seconds of brisk rubbing

00:46:15.120 --> 00:46:22.880
against a wool sleeve it will do exactly that press it lightly to the wall and it stays

00:46:23.680 --> 00:46:32.160
sometimes for hours sometimes through the night clinging with a silent steadiness the force at work

00:46:32.960 --> 00:46:41.600
is static electricity a phenomenon that reveals how tiny imbalances in electric charge

00:46:42.480 --> 00:46:51.760
can produce a noticeable pull between ordinary objects the balloon begins electrically neutral its rubber

00:46:51.760 --> 00:47:01.200
surface contains roughly equal numbers of protons which carry positive charge and electrons

00:47:02.320 --> 00:47:10.480
which carry negative charge wool like many fabrics also has a balance of charges in its

00:47:10.480 --> 00:47:19.120
resting state but when the two materials are rubbed together the contact is not gentle enough to leave

00:47:19.120 --> 00:47:28.240
everything in place electrons being far lighter and more mobile than the protons locked inside

00:47:28.320 --> 00:47:37.280
atomic nuclei can transfer from one surface to the other wool has a greater tendency to lose electrons

00:47:38.400 --> 00:47:47.200
while rubber has a greater tendency to gain them this is not a coincidence it is a predictable

00:47:47.200 --> 00:47:55.600
property that scientists have arranged into what is called the tribal electric series materials higher

00:47:55.680 --> 00:48:04.400
on the series tend to give up electrons and those lower down tend to capture them wool sits above rubber

00:48:05.280 --> 00:48:13.200
so in the rubbing electrons migrate from the sleeve into the surface of the balloon the balloon

00:48:13.200 --> 00:48:20.080
ends up with a net negative charge the sleeve is left with a net positive charge

00:48:20.720 --> 00:48:30.160
though its effect is often less obvious because the fabric is thicker and the charge may spread

00:48:30.160 --> 00:48:39.840
into the surrounding air or into moisture on the skin the balloon on the other hand is a thin insulating

00:48:39.840 --> 00:48:48.720
membrane it's newly acquired electrons cannot travel far through the rubber so they remain crowded

00:48:48.720 --> 00:48:55.920
on the surface with a rubbing occurred this is the moment when the balloon becomes capable of

00:48:55.920 --> 00:49:06.480
clinging a wall in a typical house is often made of painted drywall wood paneling or plaster none of these

00:49:06.480 --> 00:49:14.080
are strongly conductive but they are not perfect insulators either the atoms and molecules in the wall

00:49:14.960 --> 00:49:23.520
contain charges that can shift slightly even if they cannot flow freely when the negatively charged

00:49:23.520 --> 00:49:32.320
balloon is brought close to the wall its electric field pushes against the electrons in the wall surface

00:49:33.680 --> 00:49:43.440
nudging them a tiny distance deeper into the material the wall surface is left with a slight excess

00:49:43.440 --> 00:49:50.960
of positive charge a separation of charges called polarization the negative balloon

00:49:51.840 --> 00:49:59.280
and the positively polarized patch of wall attract each other the attraction is not strong enough to

00:49:59.280 --> 00:50:08.240
crush the balloon or dent the wall but it is enough to resist the gentle downward pull of gravity

00:50:08.880 --> 00:50:16.880
on the lightweight rubber the balloon remains in place because the force between the separated charges

00:50:17.840 --> 00:50:28.080
follows a pattern that the French physicist Charles Augustine de Coulomb described in the 1780s using a

00:50:28.080 --> 00:50:35.280
delicate torsion balance Coulomb suspended a small charged sphere on a thin wire

00:50:36.240 --> 00:50:43.280
and brought another charged sphere near it by measuring how much the wire twisted as the spheres

00:50:43.280 --> 00:50:53.280
repelled or attracted he was able to show that the electrostatic force grows stronger as the amount of

00:50:53.280 --> 00:51:04.720
charge increases and weaker as the distance between the charges increases specifically the force drops off

00:51:04.720 --> 00:51:12.080
with the square of the distance having the gap between two charged objects quadruples

00:51:12.080 --> 00:51:19.840
the pull between them for the balloon pressed flat against the wall the distance between the negative

00:51:19.840 --> 00:51:29.440
charges on the rubber and the induced positive charges on the wall is tiny maybe a fraction of a

00:51:29.440 --> 00:51:37.920
millimeter across the microscopic hills and valleys of the two surfaces that close proximity

00:51:38.640 --> 00:51:45.840
corresponds to a force strong enough to hold for a long time what makes the balloon eventually fall

00:51:46.880 --> 00:51:55.280
is not a sudden change in the law of attraction air is slightly conductive due to the ions that

00:51:55.280 --> 00:52:04.000
naturally drift through it created by background radiation and other subtle processes over time

00:52:04.960 --> 00:52:14.640
some of these ions bump into the balloon's surface and slowly carry away the excess electrons water vapor

00:52:14.640 --> 00:52:24.800
in the air speeds this up because water molecules are polar and can latch onto charges forming

00:52:24.800 --> 00:52:32.880
tiny conductive paths in dry indoor air the balloon may cling for many hours in humid air

00:52:33.760 --> 00:52:41.280
the charge leaks away faster and the balloon drops sooner there is no sound when it finally releases

00:52:42.240 --> 00:52:51.280
the bond simply weakens until the weight of the balloon overcomes it and it drifts to the floor the same

00:52:51.280 --> 00:53:00.160
principle shows up in other quiet ways around a home a sock pulled from a dryer might stick to a

00:53:00.160 --> 00:53:07.920
shirt for the same reason that the balloon sticks to the wall dust settling on a television screen

00:53:08.880 --> 00:53:18.720
or a computer monitor is drawn by electrostatic attraction the tiny particles polarized by the charged

00:53:18.720 --> 00:53:27.120
surface walking across a carpet and then touching a metal door knob can produce a small spark

00:53:28.560 --> 00:53:36.880
which is the sudden equalization of charge between your body and the metal these events are all expressions

00:53:36.880 --> 00:53:47.200
of the same force one that operates without motion or visible linkage simply because out of place

00:53:47.200 --> 00:53:56.320
electrons create a field that reaches across the space between objects and pulls them gently

00:53:56.320 --> 00:54:05.360
together in a quiet room a balloon on a wall is a demonstration of how balanced the world normally is

00:54:06.640 --> 00:54:14.800
and how even a tiny upset in that balance a few million extra electrons on a rubber surface

00:54:15.760 --> 00:54:23.360
can produce a force steady enough to hold a lightweight object in place silent and still

00:54:24.640 --> 00:54:33.200
until the balance slowly restores itself if you place a bar magnet on a table and lay a sheet of paper over it

00:54:34.400 --> 00:54:44.000
then dust the paper with fine iron filings something quiet and orderly appears the tiny dark

00:54:44.080 --> 00:54:54.240
grains which a moment before scattered at random now shift and settle into arcs that sweep from one

00:54:54.240 --> 00:55:03.120
end of the magnet to the other the lines are not solid threads they are a femoral formed by each

00:55:03.120 --> 00:55:12.640
filing aligning itself with an invisible influence this pattern is a map of the magnetic field a magnetic

00:55:12.720 --> 00:55:21.520
field is the region around a magnet or a magnetic force can be detected the field has both strength

00:55:22.160 --> 00:55:31.120
and direction at every point the direction is defined by the way a small test magnet such as a

00:55:31.120 --> 00:55:39.120
compass needle would align by convention the field lines run from the north pole of the magnet

00:55:39.920 --> 00:55:49.520
to its south pole outside the magnet and they continue through the inside to form closed loops

00:55:49.520 --> 00:55:58.640
inside the magnet they travel from south to north the lines never cross and they are most concentrated

00:55:59.280 --> 00:56:08.240
where the force is strongest near the poles every permanent magnet has two poles north and south

00:56:08.240 --> 00:56:16.240
bring two north poles close together and you feel a gentle but definite push apart a

00:56:16.240 --> 00:56:24.800
north pole and a south pole on the other hand pull toward each other this push and pull is the hallmark

00:56:24.800 --> 00:56:34.720
of magnetism like poles repel unlike poles attract the shape of the field lines reflects this

00:56:34.800 --> 00:56:42.880
they curve from north to south bowing outward and the density of the lines corresponds

00:56:42.880 --> 00:56:51.520
to the intensity of the field a stronger magnet produces a denser bundle of curves the most familiar

00:56:51.520 --> 00:57:01.600
magnetic field is the one we stand inside every day the earth itself acts as though it contains a large

00:57:01.600 --> 00:57:12.400
bar magnet tilted about 11 degrees from its axis of rotation a compass is simply a small lightweight

00:57:12.400 --> 00:57:20.880
magnet free to rotate horizontally it's north-seeking end points toward the earth's magnetic

00:57:20.880 --> 00:57:29.920
south pole which lies near the geographic north pole the field lines of the planet run from the southern

00:57:29.920 --> 00:57:38.320
magnetic region to the northern magnetic region arching through the atmosphere near the equator

00:57:39.040 --> 00:57:47.360
they are almost parallel to the ground near the poles they dive steeply inward this steady field

00:57:47.360 --> 00:57:55.600
guided mariners long before anyone understood what caused it the understanding grew slowly

00:57:55.600 --> 00:58:04.320
load stones naturally magnetized chunks of the mineral magnetite were known to the ancient Greeks

00:58:04.960 --> 00:58:14.160
and Chinese by the 12th century travelers used load stone slivers floated on water as crude

00:58:14.160 --> 00:58:24.320
compasses in 160 the English physician and scientist William Gilbert published a book that brought

00:58:24.320 --> 00:58:31.120
precision to the subject he carved a sphere from a load stone which he called a terrella

00:58:32.400 --> 00:58:39.040
and traced the field around it with a small compass the pattern he saw on his miniature earth

00:58:39.920 --> 00:58:46.640
matched what compasses showed on the full scale planet Gilbert concluded correctly

00:58:46.640 --> 00:58:55.360
that the earth itself is a giant magnet modern permanent magnets are made from alloys of iron

00:58:56.240 --> 00:59:06.160
nickel cobalt and other elements inside these materials tiny regions called magnetic domains act

00:59:06.800 --> 00:59:14.720
like miniature magnets in an unmagnetized piece of iron the domains point in random directions

00:59:15.600 --> 00:59:23.120
and cancel one another out when a strong external field aligns them the whole piece

00:59:23.120 --> 00:59:32.080
becomes a magnet the alignment can be locked in place creating a permanent magnet that holds its field

00:59:32.080 --> 00:59:41.200
for decades the field around such a magnet is a direct result of that collective alignment a magnetic

00:59:41.200 --> 00:59:50.960
field can be measured in units of Tesla or Gauze a typical horseshoe magnet might produce a field

00:59:50.960 --> 01:00:00.240
of a few hundred gauze between its poles while a refrigerator magnet is somewhat weaker the lines you see

01:00:00.240 --> 01:00:09.440
in the iron filing picture are not physical things but representations of the force that would act

01:00:09.440 --> 01:00:19.200
on a north pole placed at any spot the filings simply rotate until they lie along those invisible curves

01:00:20.400 --> 01:00:28.800
tracing out a shape that is always there waiting to be seen the pattern repeats faithfully around

01:00:28.800 --> 01:00:36.400
every bar disc or horseshoe magnet whether it is holding a note to a kitchen door

01:00:37.120 --> 01:00:45.920
or floating a compass card at sea it is a silent steady architecture present and unvering

01:00:47.200 --> 01:00:53.760
through which the poles reach for one another across empty space tie a stone to a string

01:00:54.880 --> 01:01:00.720
and set it swinging in a flat circle your hand feels the string tug outward

01:01:00.800 --> 01:01:08.880
but the force on the stone itself points inward towards your hand to keep the stone in its loop

01:01:09.920 --> 01:01:18.240
you must pull inward through the string just as firmly as the stone pulls outward on you that inward

01:01:18.240 --> 01:01:25.680
pull is the centripetal force its name comes from latin words meaning centerseeking

01:01:26.400 --> 01:01:35.280
because at every moment it aims straight toward the center of the circle remove the force let go of

01:01:35.280 --> 01:01:44.240
the string and the stone to something simple it is not fly outward along a curved path instead

01:01:44.880 --> 01:01:53.280
it moves off in a straight line tangent to the circle at the point of release the curve vanishes the

01:01:53.280 --> 01:02:02.720
instant the inward pull stops the same arrangement appears anytime an object moves along a

01:02:02.720 --> 01:02:11.520
curved path a car rounding a bend a planet tracing its orbit a swing at the top of its arc

01:02:13.120 --> 01:02:21.680
in each case there is a real physical force pointing toward the center of rotation the force may be

01:02:21.680 --> 01:02:28.880
supplied by friction by gravity by the tension in a cable or by the push of a wall

01:02:30.080 --> 01:02:37.520
but it always aims inward what you feel when you are the passenger in that turning car is often

01:02:37.520 --> 01:02:45.120
described as a centrifugal push pressing you into the door that feeling however

01:02:46.080 --> 01:02:53.760
is not a force acting on you from the outside it is your own bodies inertia your body

01:02:54.560 --> 01:03:02.480
following its preference to move in a straight line meets the door which is curving with the car

01:03:02.480 --> 01:03:10.960
the door then pushes inward on you supplying the centripetal force that keeps you turning remove the

01:03:10.960 --> 01:03:20.640
doors inward push let the car slide on a surface with no grip and you would continue straight ahead

01:03:21.680 --> 01:03:31.200
not be flung outward Christian hygians the Dutch mathematician and physicist gave the first precise

01:03:31.200 --> 01:03:40.640
account of centripetal force in the 1670s working with pendulum clocks and observing circular

01:03:41.200 --> 01:03:49.440
motion he worked out that the inward acceleration needed to keep an object moving at a steady

01:03:49.440 --> 01:03:59.120
speed in a circle depends on the square of its speed and the radius of the circle a faster object

01:03:59.840 --> 01:04:08.400
needs a stronger centripetal push and a tighter circle also demands more force if you shorten the

01:04:08.400 --> 01:04:17.120
string on that stone and twirl it at the same speed the pull on your hand grows if you ease your

01:04:17.120 --> 01:04:27.360
grip and let the radius increase the force becomes gentler hygians discovery became a basis for Newton's

01:04:27.360 --> 01:04:36.000
later laws of motion and gave the world a way to understand the orbits of moons and planets

01:04:36.000 --> 01:04:44.640
and everyday example sits in a playground on a spinning round about each rider is kept in a circular

01:04:44.640 --> 01:04:52.880
path by the seat or the metal bar that pushes inward if a rider shifts closer to the center

01:04:53.920 --> 01:05:02.400
the feeling of that pushed diminishes the radius is smaller so even at the same rotation

01:05:03.200 --> 01:05:09.600
rate the required inward force lessons move outward to the rim and the force grows

01:05:10.720 --> 01:05:20.240
because the circle is wider and the same angular speed means a higher linear speed for a car driving

01:05:20.240 --> 01:05:29.200
around a curve the centripetal force comes from the friction between the tires and the road the tires

01:05:29.200 --> 01:05:37.920
grip sideways and that sideward grip points toward the center of the turn on a dry textured road

01:05:37.920 --> 01:05:47.120
surface the friction supplies this inward push reliably if you steer more sharply the radius tightens

01:05:47.840 --> 01:05:55.120
and the needed force rises the tires adjust their bite accordingly if you slow down

01:05:55.840 --> 01:06:02.880
the force needed drops and the turn feels gentle engineers often bank curved roads

01:06:04.000 --> 01:06:09.520
so that a portion of the cars wait also contributes to the inward push

01:06:10.720 --> 01:06:19.840
reducing the reliance on friction alone the slight tilt of the pavement is a quiet way to help vehicles

01:06:19.840 --> 01:06:27.760
follow a curved path without extra effort from the driver perhaps the most serene example

01:06:27.760 --> 01:06:37.440
of centripetal force is the orbit of a moon around its planet gravity itself becomes the inward pull

01:06:38.640 --> 01:06:47.040
arching a straight line path into an ellipse there is no string no friction no solid push

01:06:48.000 --> 01:06:53.920
only the mutual gravitational attraction the moon is constantly falling toward earth

01:06:54.720 --> 01:07:04.400
in a sense but its sideways motion is fast enough that it keeps missing forever tracing a circle

01:07:04.400 --> 01:07:12.400
the centripetal force here is gravity and its magnitude adjusts with distance and speed

01:07:13.200 --> 01:07:21.200
just as hydrants rule describes in the kitchen a salad spinner works by the same principle

01:07:21.200 --> 01:07:28.320
the bowl is spun and the inner wall of the basket pushes inward on the lettuce leaves

01:07:29.440 --> 01:07:36.080
keeping them moving in a circle the water droplets however are not held as firmly

01:07:36.080 --> 01:07:46.400
they are small and slip through the holes continuing in straight lines once they are no longer contained

01:07:46.400 --> 01:07:54.640
the same idea scaled up operates in a centrifuge that separates liquids of different density

01:07:56.000 --> 01:08:03.680
though in that quiet context it does its work without drama so the force that keeps an object spinning

01:08:04.480 --> 01:08:14.320
is always inward always at right angles to the motion always the reason the object does not stray

01:08:14.320 --> 01:08:23.520
from its curve it is present in the slow arc of a throne ball at its peak in the hum of a bicycle wheel

01:08:24.800 --> 01:08:32.240
in the steady turning of the stars across the night sky it is a quiet dependable requirement

01:08:32.240 --> 01:08:41.600
one that never ceases until the path becomes straight again the circle holds only as long as

01:08:41.600 --> 01:08:50.320
something reaches in toward its center inside the quiet heart of every atom far from the gentle

01:08:50.320 --> 01:09:00.240
wanderings of its electrons lies a gathering of particles so small that it would take 100,000 of them

01:09:00.320 --> 01:09:07.440
placed side by side to span the width of a single atom this is the nucleus

01:09:08.640 --> 01:09:16.720
a knot of protons and neutrons held together by a force that has no equal in the every day

01:09:16.720 --> 01:09:22.320
world of pushes and poles the challenge is immediate and obvious

01:09:22.640 --> 01:09:32.080
protons carry a positive electric charge and like charges repel each other with a strength

01:09:32.640 --> 01:09:39.040
that grows rapidly as they draw near inside the cramped quarters of a carbon nucleus

01:09:40.320 --> 01:09:48.800
six protons press against one another with an electrical repulsion that would if left alone

01:09:49.760 --> 01:09:58.080
fling them apart in an instant yet the nucleus remains serenely intact the reason is a form of

01:09:58.080 --> 01:10:07.200
binding that has been called with a quiet aptness the glue at the heart of matter the strong nuclear

01:10:07.200 --> 01:10:17.040
force operates on a scale that defies ordinary intuition it reaches only across distances of about

01:10:17.120 --> 01:10:27.600
a quadrillionth of a meter roughly the width of a proton or neutron itself beyond that it vanishes

01:10:27.600 --> 01:10:36.240
so completely that an electron circling the nucleus feels nothing of it at all within that tiny sphere

01:10:36.240 --> 01:10:46.320
however it acts with an intensity that swamps the electrical repulsion many times over protons and

01:10:46.400 --> 01:10:56.000
neutrons cling to one another as if locked in a close embrace each nucleon affecting only

01:10:56.000 --> 01:11:04.400
its immediate neighbors this short-range quality shapes every atomic nucleus in nature it is why

01:11:04.400 --> 01:11:15.120
a helium nucleus with its two protons is stable and why a uranium nucleus packing many more protons

01:11:16.320 --> 01:11:23.520
heaters at the edge of stability after billions of years the story of how this force was uncovered

01:11:24.320 --> 01:11:33.200
began with a simple puzzle early in the 20th century Ernest Rutherford fired alpha particles

01:11:33.920 --> 01:11:42.800
at a thin gold foil and observed that a tiny fraction bounced back sharply that could only happen

01:11:43.600 --> 01:11:52.240
if almost all the atoms mass was concentrated in a miniscule central core the nucleus was discovered

01:11:53.280 --> 01:12:00.240
but its composition raised a new question once the neutron was identified in 1932

01:12:01.680 --> 01:12:09.520
physicists could see that the nucleus contained protons and neutrons in tight proximity

01:12:09.680 --> 01:12:20.480
yet the protons did not fly apart to explain this the Japanese theorist Hideki Yukawa proposed

01:12:20.480 --> 01:12:32.240
in 1935 that nucleons exchange a new kind of particle one that would carry the strong force between them

01:12:32.240 --> 01:12:41.840
he predicted its mass and it does in years later the pie mason or pie on was found in cosmic

01:12:41.840 --> 01:12:50.160
ray experiments confirming the basic idea today the picture reaches deeper still we understand that

01:12:50.160 --> 01:13:00.080
protons and neutrons are themselves built from smaller entities called quarks and it is between these quarks

01:13:00.880 --> 01:13:06.800
that the true strong force operates the carrier of that force the glue on

01:13:07.840 --> 01:13:16.560
binds quarks with an ever-tightening grip so that isolating one is impossible what holds the nucleus

01:13:16.560 --> 01:13:25.680
together is a residual echo of that deeper binding a kind of nuclear van der vals force

01:13:26.640 --> 01:13:34.960
leaking from the insides of protons and neutrons the strength is still enormous at close range

01:13:34.960 --> 01:13:44.800
removing a single nucleon from a medium-sized nucleus can require millions of electron volts of energy

01:13:46.080 --> 01:13:54.160
far more than the chemical bonds that link atoms into molecules because the force cares nothing for

01:13:54.160 --> 01:14:03.040
electric charge neutrons serve as a kind of extra adhesive they contribute to the strong attraction

01:14:03.920 --> 01:14:13.360
without adding to the proton repulsion which is why larger nuclei contain an increasing proportion

01:14:13.360 --> 01:14:21.280
of neutrons the intricate balance between the strong forces pull and the electric repulsions

01:14:22.240 --> 01:14:29.840
push sets the pattern of stable isotopes that fill our world the iron in a fleeting

01:14:29.840 --> 01:14:40.320
breath's oxygen the calcium in a bone the carbon in every living cell all oh their steadfastness

01:14:41.040 --> 01:14:47.200
to this hidden arrangement if the atom could be enlarged to the size of a large sports stadium

01:14:48.160 --> 01:14:56.000
the nucleus would appear as a small marble at its center still dense beyond belief

01:14:57.520 --> 01:15:06.720
a pinhead containing virtually all the atoms mass and within that marble the strong force works silently

01:15:07.920 --> 01:15:14.080
binding the pieces with an unseen hand it is a force that never reaches the skin

01:15:14.080 --> 01:15:23.520
never tugs a falling leaf or guides a compass needle yet without it there would be no

01:15:23.520 --> 01:15:33.920
complex atoms no chemistry no world of solid things the quiet permanence of matter night after night

01:15:34.480 --> 01:15:43.120
and day after day rests on this smallest of all glues the room is dark and still the blanket

01:15:43.200 --> 01:15:52.480
rests lightly over you held down by gravity which also keeps the bed frame pressed into the carpet

01:15:52.480 --> 01:16:01.120
the wooden slats beneath the mattress push upward with a spring force just strong enough to match your

01:16:01.120 --> 01:16:10.480
weight in the quiet a tiny dust mode drifts through a slant of moonlight it's fall slowed to a steady

01:16:10.480 --> 01:16:18.400
creep by air resistance every object within reach sits inside a web of these gentle

01:16:19.520 --> 01:16:28.320
balancing poles and pushes the lamp on the nightstand is motionless because friction between its base

01:16:29.200 --> 01:16:36.800
and the wood resists any sideways slide the water in a glass on the shelf stays level

01:16:37.600 --> 01:16:46.400
under gravity's steady downward tug your own body warm and heavy is held in place by tension

01:16:46.400 --> 01:16:54.240
in the sheets and the buoyant give of the pillow surrendering to sleep feels very much like this

01:16:55.760 --> 01:17:02.640
a slow release into the net of forces that have been quietly cradling you all along