Physics

Sunlight is scattered by Earth's atmosphere. Blue wavelengths (~475 nm) scatter more than other colors due to Rayleigh scattering, making the sky appear blue to our eyes.

Stars twinkle because their light passes through Earth's turbulent atmosphere, which bends the beams unpredictably. Planets twinkle far less because they appear as small disks, not points.

Cats use a built-in righting reflex, twisting their flexible spine in mid-air to rotate upright before landing.

Water expands when it freezes because hydrogen bonds lock molecules into an open crystalline lattice less dense than liquid water.

Sound requires a medium of molecules to travel through, and the near-vacuum of space has too few particles to carry sound waves.

The Sun actually emits white light, but Earth atmosphere scatters shorter blue wavelengths, making the remaining light appear yellowish.

Metal conducts heat away from your skin much faster than wood or plastic, so it feels colder even at the same temperature.

Under certain conditions, hot water can freeze faster than cold water — a counterintuitive phenomenon called the Mpemba effect, whose exact cause is still debated.

A mirror reflects light because its smooth metal coating causes photons to bounce back at the same angle they arrived, preserving the image.

Wind is caused by differences in atmospheric pressure created by uneven heating of the Earth surface by the Sun.

Magnetism arises from the alignment of electrons spinning and orbiting within atoms; like poles repel and opposite poles attract because of how their magnetic field lines interact.

Helium is much less dense than air, so sound travels about 2.7 times faster through it, raising the resonant frequencies of your vocal tract and making your voice sound higher-pitched.

Static shocks occur when electrons accumulate on your body through friction (triboelectric charging) and then discharge rapidly when you touch a conductor like a metal doorknob.

Water expands when it freezes because hydrogen bonds force water molecules into a rigid, open crystalline lattice (ice) that takes up about 9% more volume than the disordered liquid state.

At sunset, sunlight travels through much more atmosphere than at noon, scattering away shorter blue wavelengths and allowing longer red and orange wavelengths to dominate — a more extreme version of the same Rayleigh scattering that makes the daytime sky blue.

Magnets attract or repel because of magnetic fields created by the aligned motion and spin of electrons inside them; opposite poles attract while like poles repel.

Metal feels colder than wood at the same temperature because metal conducts heat away from your skin much faster; your nerves sense the rate of heat loss, not the actual temperature, so fast-conducting metal feels colder.

Birds fly in a V to save energy: each bird rides the rising air, or upwash, spinning off the wingtip of the bird ahead, and the shape also helps the flock keep sight of one another.

Earth is round because gravity pulls all of its mass towards the centre equally from every direction, and a sphere is the shape that results.

Water absorbs red, orange and yellow light more strongly than blue light. The blue light is scattered back to our eyes, so deep water looks blue.

Lightning heats the air so fast that it explodes outward in a shock wave. Sound from different parts of the long lightning channel reaches you at different times, creating the rumble.

Surface tension pulls the soap film as tight as possible. A sphere holds a given amount of air with the smallest possible surface, so bubbles become round.

The outside of the ice warms and expands faster than the frozen core, creating stress that splits it.

At cruising altitude the thin air reduces drag and saves fuel, while flying above most weather and turbulence.

The Sun’s gravity constantly pulls planets inward while their sideways motion keeps them from falling in.

Cats have a flexible spine and a righting reflex that lets them twist their body mid-air to orient feet-downward in a fraction of a second.

Walking on carpet or wearing certain fabrics causes electrons to transfer between surfaces, building up charge that discharges as a spark when you touch metal.

Sound needs a medium like air or water to travel through; space is essentially a vacuum with too few molecules to carry sound waves.

Known as Olbers’ paradox, the sky is dark because the universe has a finite age, many stars are too far away for their light to have reached us, and expansion redshifts distant light.

Helium is much less dense than air, so sound waves travel nearly three times faster through it, shifting the resonance of your vocal tract to higher frequencies.

In glass, the electrons require more energy to jump to a higher state than visible light can provide, so visible photons pass through instead of being absorbed.

Dimples create a thin layer of turbulent air that reduces drag and boosts lift, so the ball flies much farther.

The felt covering slows the ball in the air, helps the racket grip it for spin, and makes its bounce more predictable.

A moving bike steers itself into any lean thanks to its geometry and mass distribution, and the rider makes constant tiny corrections.

Snow is made of countless tiny ice crystals whose many surfaces scatter all colours of light equally, which we see as white.

Light travels almost a million times faster than sound, so the flash reaches us long before the rumble.

Ducks stay dry thanks to tightly interlocking feathers that repel water, helped by oil they spread from a gland near the tail.

Magnets attract or repel because their magnetic fields interact. Opposite poles pull together, while like poles push apart.

Magnets have two poles because their internal magnetic fields are aligned in a direction, creating opposite ends that interact with other magnetic fields.

In a vacuum, all objects fall with the same acceleration because gravity pulls on mass, while heavier objects also have proportionally more inertia. Near Earth’s surface that acceleration is about 9.8 m/s².

Gravity spreads out over the surface of a sphere, so as distance doubles, the same pull is distributed over four times the area. That makes the force drop with the square of distance.

Tides are caused by differences in the Moon’s gravity across Earth, not just its overall pull. The side facing the Moon and the opposite side both end up with tidal bulges.

Earth bulges at the equator because its rotation creates centrifugal effects that oppose gravity most strongly there. The planet’s equatorial diameter is about 43 km larger than its polar diameter.

A top loses energy to friction and air resistance, so its spin weakens. As that happens, gravity makes it wobble and eventually topple.

A gyroscope resists being tilted because its angular momentum points in a fixed direction. Changing that direction requires torque, so the response often appears as precession instead of tipping.

A superconductor expels magnetic fields, so it can lock a magnet in place through a phenomenon called flux pinning. This creates stable levitation at very low temperatures.

Magnets are dipoles because their magnetic fields are created by aligned moving charges and atomic electron spins. If you cut a magnet in half, you get two smaller magnets, each with both poles.

Earth’s magnetic field is generated by moving molten iron in its liquid outer core. That flowing, electrically conducting metal acts as a geodynamo.

Lightning travels through air in stepped bursts because air is normally an insulator. The electric field breaks the air down along the easiest ionized path, which twists and branches.

Light travels much faster than sound, so you see the flash before you hear the boom. The lightning also superheats air, creating a shock wave that we hear as thunder.

Light behaves like a wave in interference and diffraction, but like particles called photons when it exchanges energy. This wave-particle duality is a central result of quantum physics.

According to relativity, the speed of light is a cosmic speed limit built into spacetime. As an object approaches that speed, its energy requirement rises dramatically.

Light slows down in a material because it interacts with the material’s atoms, so its path changes direction. This change in speed at a boundary is called refraction.

Different colors of light bend by different amounts because they travel at slightly different speeds in glass. Shorter wavelengths like blue are refracted more than longer wavelengths like red.

Raindrops act like tiny prisms and mirrors, sending light back toward your eyes at a fixed angle. Because that angle is around 42 degrees for red light, the rainbow forms a circular arc centered opposite the Sun.

An event horizon forms because gravity becomes so strong that once you cross a boundary, even light cannot escape. For a non-rotating black hole, that boundary is called the Schwarzschild radius.

Orbiting black holes lose energy by distorting spacetime, which carries energy away as gravitational waves. As they spiral together, the waves grow stronger until the two holes merge.

Stars shine because nuclear fusion in their cores converts mass into energy. In the Sun, hydrogen nuclei fuse into helium under extreme temperature and pressure.

It depends mainly on how massive the collapsed core is after a supernova. If the remnant is not massive enough to collapse past a certain point, neutron degeneracy pressure can halt it as a neutron star.

Gravity is vastly weaker because it couples to mass-energy much more weakly than electromagnetism, the strong force, or the weak force. At everyday distances it dominates only because all mass attracts, while the other forces usually cancel out or act over much shorter ranges.

Magnets attract metals such as iron, nickel, and cobalt because their electrons can align into magnetic domains. In most other metals, those tiny magnetic effects cancel out, so there is little or no visible attraction.

A compass needle points north because it is a tiny magnet that aligns with Earth’s magnetic field. The “north” end of the needle is attracted toward the magnetic south near Earth’s geographic North Pole.

Materials conduct electricity when they have electrons that can move freely through them. In insulators, electrons are tightly bound to atoms, so electric current cannot flow easily.

Current produces heat because moving electrons collide with atoms in the material and lose energy. That energy becomes thermal motion, which is called electrical resistance heating.

Some materials become superconductors when they are cooled enough that electrons pair up and move without resistance. In that state, electric current can flow indefinitely in a closed loop.

Heating makes the atoms in a magnet vibrate more, which disrupts the alignment of its magnetic domains. If it is heated enough, it can reach the Curie temperature and lose ferromagnetism.

Electric current creates a magnetic field because moving electric charges generate magnetism according to Maxwell’s laws. The field circles the wire and gets stronger with more current.

More turns of wire create more combined magnetic field around the core. The fields add together, making the electromagnet stronger for the same current.

Steel contains iron, which is ferromagnetic and strongly attracted to magnets. Aluminum is not ferromagnetic, so ordinary magnets do not stick to it in the same way.

Charged particles bend because a magnetic field exerts a force at right angles to their motion. That sideways force changes the direction of travel without necessarily changing the speed.

A spinning top stays upright because angular momentum resists changes to its axis of rotation. Gravity tries to tip it, but the spinning motion makes it precess instead of simply falling over.

They spin faster because pulling in their arms reduces their moment of inertia. To conserve angular momentum, the rotation rate increases.

They spin differently because each body formed from separate collisions, accretion, and tidal interactions. Over time, impacts and gravity from nearby bodies can speed up, slow down, or even reverse rotation.

Tides form on both sides because the Moon’s gravity pulls more strongly on the near side than the far side, while Earth and Moon orbit a common center of mass. That creates a bulge toward the Moon and another on the opposite side.

Tidal forces pull more strongly on the near side of an object than on the far side, stretching it. If the difference is strong enough, the object can break apart.

Black holes bend light because their gravity curves spacetime itself. Light follows the curved geometry, so its path appears bent to us.

In a vacuum, all objects fall at the same rate because there is no air resistance to slow lighter or less streamlined items. Gravity gives them the same acceleration near Earth, about 9.8 m/s².

A pendulum swings at a nearly regular rate because gravity pulls it back toward the lowest point in a repeating cycle. For small swings, its period depends mainly on length, not mass.

Sound is a vibration that moves by making particles in a medium bump into each other. In empty space, there are no particles to carry the vibration, so sound cannot travel.

Rainbows form when sunlight enters raindrops, bends, reflects inside them, and bends again on the way out. Different colors exit at slightly different angles, creating the arc.

A prism splits white light because different wavelengths bend by different amounts in glass. This dispersion separates the colors into a spectrum.

Lenses bend light because their curved surfaces and different refractive index from air change the path of light rays. Convex and concave shapes make rays converge or diverge, forming images.

Light appears to slow down in glass because its electromagnetic wave interacts with the material, delaying its progress. It still travels extremely fast, but slower than in vacuum.

Lasers produce narrow beams because their light is generated by stimulated emission, which creates photons that are in step with one another. Mirrors inside the laser cavity reinforce a very specific direction and wavelength.

Earth has seasons because its axis is tilted by about 23.5 degrees as it orbits the Sun. That tilt changes the angle and duration of sunlight through the year.

Venus rotates very slowly and backward, while Uranus is tilted almost on its side, because major impacts and gravitational evolution likely altered their original spin states. These are not simple, one-time textbook examples of rotation.

Because light travels at a finite speed, about 299,792 kilometers per second, so looking far away means looking into the past.

They keep falling around the Sun because gravity pulls them inward while their sideways motion carries them forward. This balance creates an orbit instead of a crash.

Moving electric charges are electric current, and current generates magnetic fields. This relationship is one half of electromagnetism, described by James Clerk Maxwell in the 1860s.

A compass aligns with Earth’s magnetic field, not with geographic north directly. The north-seeking end points toward magnetic north, which is close to, but not exactly at, the geographic North Pole.

Dry air is a poor conductor, so electric charge does not leak away easily. That lets charge accumulate until it discharges as a shock or spark.

Heating makes atoms and molecules move more vigorously, so they take up slightly more space on average. That is why rails, bridges, and liquids expand with temperature.

Sound is a vibration passed from particle to particle, and solids have tightly packed particles that transmit that vibration quickly. Air is much less densely connected, so sound travels more slowly.

Light slows down in materials like water and glass, so its path changes direction at the boundary. That bending is called refraction.

Rainbows are made when sunlight is refracted, internally reflected, and dispersed by water droplets. The circular shape is created by geometry, but from the ground we usually see only an arc.

An object looks black when it reflects very little visible light back to your eyes. The absorbed light’s energy is usually converted into heat.

Friction turns organized motion into random molecular motion, which we experience as heat. The energy is conserved, but it changes form.

Planets are moving sideways fast enough that they continually fall around the Sun rather than straight into it. Gravity bends their path into an orbit.

Gravity is slightly different because Earth is not a perfect sphere and its mass is distributed unevenly. Elevation, rotation, and local geology all cause small changes.

The Moon’s gravity pulls on Earth’s oceans unevenly, creating two tidal bulges on opposite sides of the planet. As Earth rotates, many coasts pass through both bulges each day.

A black hole’s gravity is so strong that inside a boundary called the event horizon, even light cannot escape. The required escape speed would be greater than the speed of light.

Photons are massless particles, but they still carry energy and momentum because of relativity. That is why light can push on matter, as in solar sails.

Electric charge is a fundamental property of certain particles, built into the laws of physics rather than caused by something simpler. Charged particles interact with electromagnetic fields; neutral ones do not, or do so only weakly.

Protons all have positive electric charge, so they repel by the electromagnetic force. The strong nuclear force overcomes that repulsion at very short distances and holds nuclei together.

The strong force is mediated by particles called gluons, but its effective reach is limited inside and between nucleons. Beyond about a femtometer, the force drops off very rapidly.

Radioactive decay is a quantum process, so you can predict the chance of decay over time but not the exact moment a single atom will decay. The randomness is built into quantum physics.

Stars shine because nuclear fusion in their cores converts mass into energy. Because the process is extremely efficient and only a small amount of mass is fused each second, stars can last for billions of years.

A star’s temperature depends mainly on its mass, composition, and stage of life. More massive stars generally have much hotter surfaces and cores than smaller ones.

Dark matter is inferred because visible matter alone cannot explain how galaxies and galaxy clusters move. Its gravity appears to add extra mass that we cannot see directly.

Time seems to move in one direction because entropy tends to increase in isolated systems. The universe started in a very low-entropy state, so many processes are irreversible.

In a superconductor, electrons pair up and move through the material in a collective quantum state that avoids ordinary resistance. This happens only below a critical temperature.

Heat conduction depends on how easily energy can move through a material’s particles or electrons. Metals conduct heat well because free electrons carry energy efficiently.

Earthquakes release energy in waves that move through rock in different ways, including compressional and shear waves. Those differences arise because Earth’s materials respond differently to pressure and sideways motion.

The weak force is short-range, but it is crucial for radioactive decay and for nuclear reactions that power stars. Without it, many important transformations between particles would not happen.

Water is densest at about 4°C, so colder surface water freezes first while slightly warmer water stays below it.