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Mechanics

The slope, and the two directions that make it easy
The hill that gives it back, and the forces that do not
Turning is an acceleration, and constant speed does not help
The period that depends on the swing, computed exactly
Collisions are easier than forces, and momentum is the reason
The same push, further out, and why that is a different quantity
The force that takes what it needs
Every minimum is a parabola
The forces that are not there
The point that keeps moving as if nothing had happened
The two pendulums that will not stop swapping
The axis that will not hold
The three ways of coming to rest
The chatter a stiffer holder removes
The energy that depends on the observer
The length nobody has to measure
The push that needs nothing to push against
The error that doubles on a schedule
The last curve to go
The pile that lands heavier than it weighs
The angle that drag moves
Least action, except that it is not least
The conservation law a symmetry hands over
The fold that has to be there
The curve that does not ask where it started
The force a coordinate cannot see
Everywhere a throw can reach
Held up by a force that averages to nothing
Slide or topple
The angle that throws furthest, and why nobody notices
The grip that is not a coefficient
The pendulum, and the small lie that makes it simple
The table statics cannot settle
The mass, and where it sits, which is what decides the race
The push that comes out sideways
The quantity that survives a change of shape
The top that nods before it settles
Five balls, and the law that does not choose
The part of the wrap that is actually gripping
The principle that fixes the energy instead of the clock
The map a dripping tap turns out to be
The oscillator that answers at three times the question
The grip that needs a little slipping
The axis a leak of energy chooses
The deflection that closes on itself
The ratio that decides whether the planet is turning
One curve answers every slope
The best throw is a tangency
The speed at which grip hands over to power
The calm that is the ghost of a cycle
The load nobody applied
A state no load could reach
The one number the tolerances cannot touch
The floor that does no work
The wall that moves while the ball is in flight
The throw most likely to go in
The action that knows where every path ends

Waves

When two waves meet, they simply add
Only some notes fit, and that is where discreteness comes from
The note that changes on approach, and the two ways of getting it
The frequency that gets an answer, and the quarter cycle nobody mentions
A wave is a shape that travels, and nothing else does
The medium decides the speed, and the source only decides the note
Every front is a source
What happens where the medium changes
The equation that lets a shape travel
The drum that has no harmonics
The swing that is pumped, not pushed
The correction that took a century
The packet that will not keep its shape
The speed that carries no signal
The pipe that will not carry a low note
The gap a repeat opens
The channel with no walls
The mode that lives in the mistake
The distance that takes the treble out
The cone the source leaves behind
The taper that matches every note
The width that is a lifetime
The answer that cannot come first
The arrival that keeps arriving
How many ways there are to vibrate
The front that steepens until it cannot
The pulse two failures keep alive
The wave that comes from the rim
The wave a surface is enough to hold
The count that cannot be cheated
The shift a mirror gives twice
Swap the ends and nothing changes
How a wave thins out
The layer that makes a reflection vanish
The mass that makes another stand still
The packet that moves at another speed than its own crests
The spiral that says how much light arrives
What adding does to the energy
The resonance with a zero in it
The backward wave Huygens had to remove
The frequency a lattice cannot carry
The node that is not standing still
The mode that will not turn a corner
The dent that raises the note
Where the loudness goes
The steps in an absorption curve
The end that knows how the middle was cut
The mirror that works from every direction
The resonance that refuses to leak
The mismatch no network can remove
The fan of plane waves inside every beam
The one medium that was supposed to add exactly
Two tails that swap everything
The ripple that counts the neighbours
Below the gap, where there is nothing to absorb
The solitons a hump already contains
The coupler that does not care about the colour

Optics

What a lens is doing, and why three rays are enough
The angle past which light cannot leave
The angle the rainbow has to be, and why nobody chose it
The mirror that cannot focus, and the shape that can
Where rays stop being enough, and a shadow acquires a bright centre
The path that does not change
How far apart two things have to be
Why two lamps never interfere
What a thousand slits buy that two cannot
The angle at which reflection picks a side
The ray that bends without a surface
The brightness no lens can increase
The rotation a return trip doubles
The grating that photographs itself
The image that is a diffraction pattern twice
Why a litre of water is not blue for the reason the sky is
The wavelength a fibre does not smear
The condition a lens must meet
The fringe that measures a star
The fringes below the rainbow
The grain that is in the light
The retarder with no crystal in it
The lens that is a set of rings
The correlation that survives what the phase does not
The law that only asks about one component
How far a wave can remember
The bend at the boundary, and what it is really about
The ring at twenty-two degrees
The path that takes the longest time
The crystal that answers twice
The direction of the shaking, and the filter that only asks about it
The phase that is only a shape
The reflection that happens where the glass is not
Two glasses that cancel a derivative
When the particle is the size of the wave
Why the sky is blue and the sunset is not, from one exponent
The cone a fibre will accept
The focus that is a slab, not a plane
The light with no direction of shaking
Everything a scatterer removes, from one direction
The surface that images one point exactly
The fringe and the spectrum are one measurement
The flat scene that comes back curved
The pattern the sky is written in
The angle that is two angles
The ray on the wrong side of the normal
The cone light has to find to get out
The delay that is a random variable
The cloud light has to walk through
The reflection that needs no surface
The invariant that is a count
The work a diluted beam will not do
The same cone, and a different arrival
The rings that belong to the edge
How accurate a mirror has to be
The crystal made of moments
The walk that interference can stop

Relativity

Now is a choice of slicing
Two axes, one speed, and a diagram that does the arguing
The length that depends on when, and is not really about length
Speeds that refuse to add, and the quantity that does
The twin who comes back younger
The shift that survives at right angles
The quantity nobody argues about
Magnetism is electricity seen sideways
The pole that fits and does not fit
The invariant that survives a boost
The sky that crowds into a cone
The push that does not point where the body goes
The field nobody can transform away
The string that breaks between two rockets
The box of light that weighs something
The ring where the two beams disagree
The wall of silence behind a rocket that never stops
The two clocks that flew in opposite directions
The ship that never arrives at c
The temperature of an acceleration
The speed that cannot be measured one way
The motion that measures faster than light
The brightness that is not the same for everyone
The disc that cannot be spun
Everything from an exchange of pulses
The clock that does not feel the turn
The clock that has to slow, and why no clock can refuse
The collision that wastes most of the energy
Mass is a form of energy, which is not the same as a source of it
The clock that is wrong in two directions
The contraction no photograph shows
The mass that is missing
The turn that two pushes leave behind
Which came first, and who decides
The diagram a ruler cannot read
The centre that is not a place
The one quantity a boost leaves alone
The space that speeds live in
The longest way round is the shortest clock
The drag that was only an addition
The cone a decay cannot leave
The transformation that never mentions light
Charge and current are one thing
Six numbers, one object
Nothing is allowed to be rigid
The fuel a starship needs
What the light cones alone can decide
The five places infinity turns out to be
The slope a spin leaves in a spectrum
The plane in which three bodies are flat
The rocket that leaves its fuel at home
How big now is
The body that has no temperature when it moves
The count that no observer can disagree about
The bath that pushes back
The orbit that ages less than a throw
The column that is hotter at the bottom

Thermodynamics

The speeds in a still room
Entropy is a count, and the arrow of time is arithmetic
The ceiling on every engine, set before it was designed
The equation that only runs forwards, and the walk underneath it
How far a molecule gets
The entropy that lives on a surface
Half a kT for every way of moving
The exponential that decides everything
What a system actually minimises
The viscosity that does not care how much gas there is
Why heating a perfect spring changes nothing
The point at which the two become one
The bit that has to be paid for
The glow that says nothing about the surface
The share that is not half a kT
The staircase that never reaches the floor
The entropy that is still there at zero
The gas that leaves is not the gas inside
The barrier a new phase has to climb
Hotter than any temperature there is
The gas that cools by being let go
The gas that nobody counted
The second experiment that cannot disagree
The summer that reaches the cellar in December
Why the triple point is a point
A boiling point is a pressure, not a temperature
Pressure is a rate of arrival, and the gas law falls out of counting
The jiggle that proved atoms
The transition with nothing to order
The walk that comes home
The engine that has to finish
Mixing what is already mixed
The engine that pays back more than it takes
The heat that changes no temperature, and where it actually goes
The part of the curve no fluid follows
Why the air thins with height, and why that is the same law as the speeds
The temperature a molecule does not have
The melting curve that leans the wrong way
The gradient that drives the other thing
The second law, with a probability attached
The first correction to the gas law
The engine a fluctuation cannot run
The energy that refuses to be shared
The height a planet is seen from
The entropy that depends on how fast it was cooled
A law about spectra, not about heat
The temperature an engine really takes its heat at
The work left in two buckets of water
The count that decides which entropy is right
The gas that flows towards more of itself
The condition three modes never meet
Half a kT in a piece of wire
Weighing what cannot be put on a scale
A fridge with no work going into it
An engine with one number in it
The reaction that cannot go all the way
The glow that carries a voltage

Electromagnetism

The shape decides the falloff, and the force law never changes
The inside of a conductor, where the field is exactly nothing
The field that makes the other, and only while it is changing
The term that made light
Where the energy of a field actually is
The charge that has to be somewhere else
The magnet that has to fight its own field
The field that points where the charge is now
The angular momentum that is in nothing at all
The constant that depends on how fast it is asked
The field outside the solenoid, which is not zero
The potentials that are not unique
How far a field gets into metal
The law that is always true and rarely useful
A refraction with no wave in it
The two in the flux quantum
The two equations that are not laws of motion
The pressure a charge puts on its own metal
Counting what comes out, and never looking inside
Field lines are a choice, not a discovery
Nothing can be held still by a static field
One number for every point, and nothing at all is lost
The attraction that needs no charge
The coupling that is the same both ways
The field before the lines were drawn on it
The field that wraps a current
The field the matter takes away
The field with no ends, and the force that does no work
The force that does no work
The force that lives where the model is not
The loop that behaves like a needle
The magnetism classical physics forbids
How much charge a shape will hold, before anything is charged
The magnet that falls slowly
The rule that is two laws wearing one coat
The circuit that fights its own change
The resistance that is a length
The symmetry one missing charge would complete
Two lengths, and which one is longer
The momentum of something that is not moving
The curve that is really a staircase
The force read off a surface that touches nothing
The drift that does not care what the charge is
The distance where a field changes its mind
When the source is not heard all at once
The solution that is thrown away
The field that is pushed out
The pull that grows on the way down
The voltage that is a frequency
The potential is where the wanderers stop
What holds a magnet together is not magnetism
The first length that belongs to the substance
Nothing keeps a magnetisation for ever
The field that points against the magnet it is in
A potential that does not come back to itself
The field outside that cannot find the core
The current no particle carries
The circuit that forgets its charge

Quantum

The curve that would not come down
Light arrives in lumps, and brightness only changes how many
A photon with a momentum, and a collision that proves it
Everything has a wavelength, and almost nothing shows it
The box that allows only some energies
The spectrum is a subtraction, not a list of values
Where the electron probably is
Sharpness has to be paid for
The wall that is not quite a wall
The answer that was not there before
No two in the same state, and why matter has volume
What happens when the wells get close
A nucleus with no clock
Where the quantum picture hands back the old one
The correlation no instructions can produce
A wall that a factor of two makes impassable
The angular momentum that is not a rotation
The measurement that never touched it
The disagreement that one run settles
The line that is really two
The exponential that is only true in the middle
Four states, and one of them is odd
The order the shells fill
Where the interference goes
The experiment a wave cannot pass
The crossing that never happens
The force with no force in it
The last atom does all the seeing
The state that cannot be copied
The average that obeys Newton
Two walls that let more through than one
One level, and the field that bends the bands
One arrival at a time, and the pattern still appears
The liquid that will not slow down
The mass a curve decides
The motion that cannot be stopped
The pressure that is not a temperature
The questions that can be asked together
The phase a magnet leaves on a path it never touched
The state that swings like a pendulum
How long the crossing takes
What two have they cannot give a third
Why an atom is the size it is
The turn that has to be made twice
The energy that did not all arrive
The chain that runs at its slowest member's rate
The half-life that chemistry can change
The return a classical cloud never makes
The probability that goes below zero
The field an atom calls strong
The fastest a state can stop being itself
A link between two that never met
The corner of Hilbert space that is ever visited
Two states where the counting says three
The experiment that defines spin and cannot be done on it
The outcomes identical photons refuse
The noise pushed below the floor
The fall that leaves the mass in the phase

Fluids

The pressure that only knows depth
The weight of the water that is not there
How high water will climb
The angle a liquid makes with what it sits on
The surface a spin decides
The block the water does not lift
The silo that does not weigh what it holds
The pressure that comes from counting
The column that is pulled, not pushed
The layer a parcel cannot leave
The wave that picks an angle
The angle that does not know the size of the heap
What it costs to take the salt out
The angles a film has no choice about
The big one comes to the top
The skin that is not a skin
Force multiplied, and nothing gained
Momentum going sideways
The fluid that answers back
The fountain a lamp can drive
The fourth power in a pipe
The heap that becomes a solid
The heat that arrives as a wave
The liquid that climbs the rod
The liquid that remembers
The paste that holds up its own hill
The small bubble blows up the big one
The speed that depends on the length
The thickness that goes both ways
The thread that cannot stay a thread
The whirlpool that comes in one size
Why a ship comes back upright
The depth past which it must sink
The height a siphon cannot pass
The ring the drop leaves behind
The surface that pulls toward the stronger side
The shear that only reaches so far
The push that has no direction
The hourglass that keeps time
The membrane that almost holds
The corner a liquid never stops climbing
The swelling a membrane cannot stop
The film that goes black before it bursts
The pile that is never finished settling
The wave that is required to stand still
The reflection that changes the wavelength
The pore that fills from dry air
The angle a voltage can set
The stretch a chain cannot outrun
The counterions that never leave the chain
The latitude past which a tide cannot split
The body that displaces two things
The drift a sound leaves behind
The oil that is a glass for a quarter of a millisecond
Whether a fluid can be made arbitrarily thin
The wave that holds a ship back
The like charges that pull together

Astrophysics

The floor that cannot be told from gravity
The clock that runs slow lower down
Light has a pressure
The field that cannot get out
The size at which a body becomes round
A charge that turns must glow
Where every model runs out at once
The orbit that cannot be made smaller
The orbit that has to shrink
The ball of gas that heats up as it cools
The lens with no focal length
The knot the field cannot untie
The parallelogram that will not close
The force a charge exerts on itself
The mass no cold matter can hold up
The brightness a mass cannot exceed
The disturbance that grows instead of travelling
The distance that forgets the moon
The circle light cannot leave
The wall a plasma builds against itself
The light that pulls rather than pushes
The frequency below which nothing gets in
The light that takes a hundred thousand years to leave
The long-range force that does not reach
The wave that dies with nothing to rub against
The whistle that arrives sorted
Two clocks that disagree about the fall
How far a neutrino gets
The bend Newton got half right
The delay that is not a bend
The horizon that nothing marks
The orbit that does not come back to itself
The size the light cannot blow away
The surface that only lets things in
The term free fall cannot remove
The wave that stretches one way and squeezes the other
The arrow that says which way the orbit points
What the instrument actually hears
The hole that outlives everything and then does not
Whether a charge on a table glows
The length no experiment can resolve
The area that is not allowed to shrink
The clock that measures a height
The fall that does not depend on what is falling
The same force whichever way the surface faces
The wave that does not know what the gas is made of
The limit that belonged to a simpler atom
The friction made of light
The bill that arrives when the pushing stops
The binding energy that has to fall too
The bias no battery supplies
The estimate that misses by a hundred and twenty
The scale that may not be where it looks
A few cycles that are only mass and spin
The ring that does not come back
The twist that outlives the turbulence
The clocks that must all slow together

Illustrated Physics

An illustrated collection of essays about the physics that is best understood by looking at it — every figure generated from the equations it describes.

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Themes

What stays the same
The same equation again
Approximations that lie
Fields, not forces
The arrow of time

Every figure here is drawn from scratch, in code, and rendered as SVG at build time.