Learn Physics
Short concept articles with links to try the ideas in interactive simulations.
Concept articles
Projectile Motion Explained
When you throw a ball, kick a soccer ball into the air, or fire a cannon, the resulting path is one of the most recognizable shapes in physics: a parabola. Projectile motion analyzes this by splitting the object's motion into two completely independent pieces — a constant horizontal velocity, and a vertical velocity that gravity steadily accelerates downward. Neither piece knows about the other; treating them separately, then recombining the results, is what makes it possible to predict exactly how far a projectile will travel, how long it stays in the air, and how high it climbs, using nothing more than basic kinematics.
Ohm's Law and DC Circuits
Every circuit you build — from a simple flashlight to a phone charger — turns on the same three quantities: how hard the battery pushes (voltage), how much charge flows as a result (current), and how strongly the components in the way resist that flow (resistance). For the ohmic parts, Ohm's law, V = IR, ties them together with a proportionality simple enough to predict circuit behavior with nothing more than arithmetic, and it underlies every more advanced circuit-analysis technique that follows, from series and parallel networks to the AC circuits used in real electronics.
The Photoelectric Effect
Shine light on a metal surface and, under the right conditions, it will fling out electrons — a stubbornly strange result once classical physics tried to explain it. Nineteenth-century wave theory predicted that dimmer light should still eject electrons, just more slowly, given enough time to build up energy. Real experiments showed the opposite: below a sharp cutoff frequency, no amount of intensity or waiting produces a single electron, while even the faintest light above that frequency ejects electrons instantly. Einstein's 1905 explanation — that light itself arrives in discrete packets of energy, later called photons — resolved the puzzle and became one of the founding results of quantum mechanics, for which he won the 1921 Nobel Prize in Physics.
Standing Waves on a String
Pluck a guitar string, blow across the mouth of a bottle, or shake a jump rope at just the right speed, and you'll see the same phenomenon: two waves traveling in opposite directions along the same medium interfere to create a pattern that appears to stand still rather than travel. This happens whenever a wave reflects off a fixed or free end and overlaps with its own reflection — at most frequencies the result is a messy jumble, but at specific resonant frequencies the interference locks into a clean, stationary pattern of points that never move (nodes) and points that swing between maximum displacement (antinodes). Nearly every musical instrument, from strings to wind instruments to drumheads, relies on exactly this behavior to produce a stable pitch.
Buoyancy and Archimedes' Principle
A massive steel cargo ship floats while a small steel coin sinks — same material, wildly different outcomes, and the explanation has nothing to do with weight alone. Any object submerged in a fluid pushes some of that fluid out of the way, and the fluid pushes back with an upward force exactly equal to the weight of whatever it displaced — a relationship discovered over two thousand years ago by Archimedes and still taught, unchanged, as one of the oldest quantitative laws in physics. Whether that upward push is enough to support the object depends not on its weight alone but on its shape: a ship's hull displaces a huge volume of water for its weight, while a solid coin displaces very little.