Physics

Not the chair, not this page

You Have Never Touched Anything

Solid contact is a force, not a meeting — the outer electrons of two surfaces repel each other hard enough that nothing ever quite arrives.

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A skin made of pulling

Why Does a Paperclip Float on Water?

Water molecules cling to one another so strongly that the surface behaves like a stretched skin, and a paperclip simply rests in the dent.

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Waves bunched and stretched

Why Does a Siren Change Pitch as It Passes?

To the driver the siren never changes at all. Light does the same thing, which is how a receding galaxy shifts red and betrays an expanding universe.

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Light changing lanes

Why Does a Straw Look Bent?

Light changes speed crossing from water into air, and changing speed at an angle makes it turn. Your brain assumes straight lines and misplaces the straw.

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About a hundred watts of it

You Are Glowing Right Now

Warm objects radiate light whose colour depends on their temperature — yours is infrared, invisible to us and perfectly obvious to the right camera.

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You could not start, either

What If Friction Stopped for One Minute?

Everyone expects to slide. Almost nobody expects to be unable to start, hold, stand, or keep a single screw or knot from letting go.

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An invisible ocean

Air Is Pressing on You

Ten tonnes of air is pressing on you right now and you feel nothing, because it presses equally from every side. Break that balance and you notice.

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A hidden middle

What Keeps Things From Falling Over?

Every object has a balance point. It stays upright only while that point sits over its base, which is why a wide base and low weight rarely tip.

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Pressure from below

Why Does Water Push Up?

Water pressure is stronger the deeper you go, so the push on an object's underside beats the push on its top. That leftover push is what holds it up.

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Climbing without a pump

Why Does Water Climb Up a Paper Towel?

Look at the curved edge where water meets a glass. A paper towel is that same edge repeated through countless tiny channels, all pulling at once.

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Passing motion along

What Happens When Things Crash?

Motion is never destroyed in a crash, only handed over. What decides the damage is not the speed itself but how long the stop is allowed to take.

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Light, sorted

Where Do Colours Come From?

White light is every colour at once. An object's colour is the one slice it refuses to absorb and throws back at your eyes — the leftovers, in other words.

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Jostling passed along

Why Does a Metal Spoon Get Hot?

No atom ever travels along the spoon; each stays put and passes the jiggle on. Metal feels cold only because it pulls heat out of your hand fast.

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Heavy for its size

What Is Density?

Density is not weight but how much weight is packed into a given space. It is what decides whether a thing floats, sinks, or rises through the air.

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It never goes anywhere

Where Does Energy Go When Things Stop?

Energy is never used up when something stops. It has only changed form, and every change spills a little as heat that is hard to use again.

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Bouncing all the way

How Does Light Go Round a Bend?

Light strikes the inside wall of a glass fibre so shallowly that it reflects perfectly, thousands of times a metre, tracing the cable round bends.

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Water pushes back

Why Do Things Float?

You float when the water you shove aside weighs more than you do. That is why a steel hull floats where a solid steel lump sinks straight to the bottom.

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Why things stop

What Is Friction?

Every surface is rough close up. Friction is those roughnesses catching and grinding on each other, quietly turning motion into heat as they go.

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Trading speed for strength

How Do Gears Work?

Count the teeth and you have the ratio. Ten driving thirty means three turns for one, and three times slower is three times the turning force.

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The pull you never notice

What Is Gravity?

Every object with mass pulls on every other one. Earth simply has enough mass for its pull to dominate everything you have ever felt.

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Everything is shaking

What Is Heat?

Heat is not a substance you add. It is how fast the atoms are jiggling, which is why a thermometer is really a gauge of atomic speed.

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Rømer, 1676

How We First Timed Light

Light was thought to be instant until Jupiter's moon kept running late, arriving behind schedule whenever Earth was further away.

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Cavendish, 1798

How Anyone Weighed the Earth

Henry Cavendish weighed the Earth by measuring the faint pull between four lead balls, in a sealed shed he watched through a telescope from outside.

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Water breaks the rules

Why Does Ice Float?

Almost everything shrinks as it freezes. Water does the opposite, locking into an open, gappy lattice that takes up more room — so the solid floats.

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The answer is: nothing

What If Everyone Jumped at Once?

If everyone on Earth jumped together, the planet would shift by less than the width of an atom — and then shift straight back.

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You would not get stronger

What If Gravity Were Half as Strong?

Your jump would double and nothing about you would improve: bones weaken, trees overreach, and the planet slowly leaks its air away.

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You would come back

What If You Fell Through a Hole in the Earth?

A hole through the Earth is a pendulum. You speed up to the centre, slow down to the far side, and it takes 42 minutes each way.

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Nothing changes for free

Why Do Things Keep Moving?

An object keeps doing exactly what it was doing until something makes it stop. Every change of speed or direction has to be paid for with a force.

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It makes nothing — it slows

Does a Coat Make You Warm?

A coat makes no heat of its own. It traps still air, which slows the escape of the warmth your body is already producing every second.

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Light marching in step

What Makes a Laser Different From a Torch?

A torch throws every colour every way with the waves unrelated. A laser is one colour, one direction, and every wave stepping in time.

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Bending light on purpose

How Do Lenses Work?

Curved glass slows light by different amounts across a beam, and uneven slowing makes the beam turn — towards a point, or away from one.

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Trading distance for force

How Do Levers Work?

A lever lets a small push move a big load, by making you push a long way to shift it a short one. Force and distance always trade off against each other.

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How big the push is

What Makes a Sound Loud?

Loudness is how hard the air gets shoved; pitch is how often. The two are entirely independent, which is why a whisper and a shout can hit the same note.

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Invisible reach

How Do Magnets Work?

Iron is already full of tiny atomic magnets, and magnetising it only lines them up. That is why cutting a magnet in half gives you two whole magnets.

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Our planet is a magnet

Why Does a Compass Point North?

Molten iron churning deep inside the planet generates a magnetic field reaching out into space. A compass needle is just a small magnet lining up with it.

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A very smooth wall

How Do Mirrors Work?

Every surface bounces light. A mirror is simply flat enough to bounce every ray without scrambling the arrangement, so the picture survives the trip.

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Mass times speed

Why Are Heavy Things Hard to Stop?

Nothing is lost when things collide: what one object loses another gains. Airbags, crash mats and bent knees simply make the stop take longer.

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Air shoved out of the way

How Does a Parachute Slow You Down?

A parachute does not slow you by being large. It forces you to shove a huge amount of air aside, and the air pushes back until it balances your weight.

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A reliable swing

Why Does a Pendulum Keep Time?

A pendulum's swing takes the same time whether it swings wide or narrow, because the period depends on the string's length and nothing else.

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Counting wiggles

What Makes a Sound High or Low?

Fast vibrations sound high and slow ones sound low. That is the whole of pitch — a count of wiggles per second, and nothing more complicated.

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Force, spread out

What Is Pressure?

The same push feels utterly different depending on the area it lands on. Shrink the area and you have a drawing pin; spread it out and you have a snowshoe.

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Sharing the load

How Do Pulleys Help?

Run the rope through more pulleys and the load is shared across more sections, so each pull is lighter. You pay for it by pulling a great deal more rope.

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Newton's third law

Every Push Has a Push Back

You cannot push anything without it pushing back exactly as hard. That pair of forces is what makes walking, swimming and flying possible at all.

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Same family, different size

Is Radio Really a Kind of Light?

Radio is genuinely light, just stretched to waves metres long instead of fractions of a millimetre. That length is why it passes through walls.

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The same climb, stretched out

Why Are Ramps Easier?

A ramp does not reduce the work of getting up there. It spreads the same climb over a longer walk, trading a hard short lift for an easy long push.

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Light coming back

Why Can You See Yourself?

Light bounces off a smooth surface at exactly the angle it arrived, and your brain reads the returning rays as an object standing behind the glass.

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Pushing in time

Why Does One Note Rattle the Window?

Every object has a rhythm it swings at naturally. A sound matching that rhythm adds a push on every swing, so tiny pushes build into a rattle.

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Light in a straight line

Why Do Shadows Happen?

Light travels in straight lines and cannot bend around things. A shadow is simply the gap left where something stood in the way of those lines.

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Sun overhead

Why Are Shadows Short at Noon?

Shadow length is a reading of the Sun's height, nothing more. The Sun is highest at midday, so that is when your shadow is at its stubbiest.

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A thin film of water

Why Is Ice So Slippery?

Ice carries a film of liquid water well below freezing, because its top layer of molecules has nothing above to hold it in place. That film is the slip.

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Air, pushed

How Does Sound Travel?

Sound is not a thing that flies. It is a squeeze handed from one air molecule to the next, passed all the way along to your eardrum.

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Fifteen times faster through steel

Sound Goes Faster Through a Wall Than Through Air

Sound travels by particles shoving their neighbours, so the stiffer the material the faster the shove passes — steel beats water, and water beats air.

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Squeeze and stretch

What Does a Sound Wave Look Like?

Each patch of air only shuffles back and forth a fraction of a millimetre, passing the squeeze along at 343 metres a second. Nothing travels with it.

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Nothing is pushing it out

Why Does the Water Stay in the Bucket?

Nothing flings the water outward. It never turns at all — the bucket keeps curving into its straight path and shoving it inward, over and over.

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Storing a push

How Do Springs Work?

A spring stores the work you put into squashing or stretching it, then hands that energy straight back as it returns to its own shape.

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Rubbed the wrong way

Why Do You Get Shocks?

Rubbing moves electrons by the billion, and they sit on you until the pile-up can rip air apart to escape. A door handle can hit 3,000 volts harmlessly.

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Empty space cannot push

What Does Nothing Actually Do?

Air strikes every square centimetre of you with about a kilogram of force, from all sides at once — which is why a can crumples and you do not.

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Wobbling back to the middle

Why Do Things Vibrate?

Push something springy out of shape and it overshoots the middle coming back, then overshoots again. Match its natural rate and the swing keeps growing.

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Rolling beats dragging

Why Are Wheels So Good?

A wheel does not slide against the ground. Its contact patch sets down and lifts off again, which replaces sliding friction with very nearly none.

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