Simulations

Spin a platform up, then pull two hand-held masses in toward the axis and watch the spin rate jump — angular momentum stays fixed the whole time, so pulling the masses in tighter is the only thing driving the speed-up.

Hang two masses from a string over a pulley — adjust each mass, gravity, the pulley's own mass, and rope/pulley friction to see which side descends, how their tensions differ, and watch the balanced case where equal masses simply hang still.

Heat an object toward a chosen target temperature and watch it glow the colour a real blackbody emits — deep red to blue-white — while its radiated power rises steeply with temperature.

Fire a charged particle into a uniform magnetic field (a closed cyclotron circle), add a uniform electric field (a bending parabola), or combine both to see the guiding center drift sideways — adjust charge sign, mass, speed, and both fields.

Send two disks toward each other with a chosen impact parameter — adjust their masses, starting speeds, and how off-center the hit is to see a head-on collision exchange velocities and a glancing one deflect both disks sideways, momentum and kinetic energy conserved exactly.

Fire an X-ray or gamma photon at an electron and watch it bounce off with less energy — the wavelength shift depends only on the angle, which is the evidence that light comes in particles.

Fire a charge into a uniform magnetic field, perpendicular to its velocity — adjust the entry speed, field strength, and charge-to-mass ratio to see the radius and period of the resulting circular orbit.

Watch a disease spread through 1000 people shown as 400 dots, and see how vaccination stops it. Choose how many others each case infects (R0), how many days people stay infectious, how many people are vaccinated and how many are infected on day 0. The dots turn from blue to red as the infection spreads outward from the first cases, then to green as people recover and become immune, while vaccinated people carry a ring and are never infected. A graph follows the share of people who are not yet infected, infected and recovered, and a gauge marks the herd immunity level, 1 - 1/R0 (for instance 1 - 1/3 = 66.7 % when R0 is 3), above which the disease can no longer spread. The results give the peak share infected and the day it happens, the total who catch it and how long the outbreak lasts.

Fly a sound source past a fixed observer — adjust its speed, true pitch, the speed of sound, and how close it passes to hear (and see, in the compressed and stretched wavefronts) the classic pitch glide, and watch a Mach cone form once it goes supersonic.

Hinge a second rod off a swinging pendulum's bob and release both from a large starting angle — watch a genuinely chaotic system where a fraction-of-a-degree difference in the start sends it on a completely different path.

Shine coherent light through two narrow slits and watch the exact bright/dark fringe pattern form on a screen — adjust wavelength, slit separation, slit width, and screen distance to see the fringes tighten, widen, and even lose whole orders to the single-slit diffraction envelope.

Push a damped mass on a spring with a sinusoidal driving force — sweep the driving frequency through the natural frequency and watch the transient build into a steady-state oscillation, tracing out the resonance curve as you go.

Watch an electric current split a compound apart. Two electrodes dip into a liquid and are joined to a DC supply: positive ions drift to the negative electrode (the cathode) and gain electrons, negative ions drift to the positive electrode (the anode) and lose electrons, and the products appear as bubbles or as a growing layer of metal. Choose molten lead bromide, copper sulfate solution with inert or copper electrodes, concentrated sodium chloride solution or dilute sulfuric acid, then set the current and how long it flows. The half-equation for each electrode is shown, with the charge passed and the amount of each product worked out from Faraday's laws, for instance 1 A for 20 minutes deposits 0.395 g of copper. With copper electrodes the anode loses exactly the mass the cathode gains, and dilute sulfuric acid gives hydrogen and oxygen in a 2 to 1 volume ratio.

Let a reversible gas reaction settle into equilibrium, then disturb it — add a reactant, remove product, compress or expand the vessel, or heat or cool it — and watch the concentrations shift and resettle into a new equilibrium, with a visible colour change for the nitrogen dioxide reaction.

Slide a bar magnet through a coil — adjust its speed, the coil's turns, and the magnet's strength to see the induced EMF rise, peak, flip sign as the magnet passes through, and fade as it recedes.

Pick a metal for each half-cell and set the ion concentrations, temperature and load, and see which electrode is the anode and which the cathode, the cell voltage from the Nernst equation, and electrons moving through the wire while ions move through the salt bridge. As the cell runs, the voltage falls and the two electrodes change in mass by different amounts.

Pick a type of cross and the genotype of each parent, and see the exact Punnett square, the expected ratio and a random family of offspring drawn from it. Compare the counts with the ratio using a chi-squared test, and see how far chance moves a small family compared with a large one.

See how greenhouse gases keep the Earth warm, using the energy that flows in and out. Sunlight arrives, some of it bounces straight back, and the rest warms the ground. The warm ground gives off infrared, and gases in the air absorb some of it and send part of it back down, so the ground ends up warmer than it would be without them. Arrows show each flow in watts per square metre, drawn to one scale, and a graph follows the surface temperature over 25 years. Set the carbon dioxide level (180 to 1,200 ppm), how much sunlight is reflected, whether the water vapour feedback is on, and whether the air starts as it was before industry or as it is today. For instance, with the starting settings the surface is 14.0 °C at 280 ppm, and without any greenhouse gases it would be about −18.6 °C. It is a simple model with one layer of air, so its numbers are a guide, not a forecast.

Move a spacecraft from a low orbit to a higher one with two engine burns — see the half-ellipse it coasts along, read both speed changes, and find out why the cheap route is also the slow one.

Hang weights on a spring one at a time and watch it stretch, bounce and settle, while a graph of pull against stretch is drawn point by point. Within the spring's limit the points fall on a straight line: double the pull and the stretch doubles, and the slope of the line gives the spring's stiffness. Keep adding weight and the line bends, and when the weights come off the spring stays longer than it started. Compare one spring, two joined end to end and two side by side, and change the stiffness, the weights and the limit.

Live interactive gas cylinder — set the amount of gas, then drag temperature and applied pressure and watch the piston respond in real time as PV=nRT holds exactly.

Watch a planet trace an exact elliptical orbit with the star at one focus, its equal-time sectors shaded to show equal-area sweeps, and a second comparison planet whose period and semi-major axis satisfy the same T^2/a^3 ratio.

Drive a series RLC circuit with an AC source — sweep the drive frequency through the resonant frequency and watch the steady-state current amplitude peak, with the peak's sharpness controlled by the circuit's resistance.

Pick a real gas and sweep its temperature, and watch the curve of how many molecules move at each speed shift, broaden, and flatten — track the shaded fraction moving faster than a chosen speed threshold as the temperature climbs.

Watch a current-carrying conductor feel a push in a magnetic field. In one view a thick wire hangs between a north and a south pole; switch on the current and it swings out, with arrows for the field, the current and the force and the force worked out as F = B × I × L (for instance 0.4 T × 3 A × 0.1 m = 0.12 N). In the other view a coil sits on an axle between the poles with a split-ring commutator that reverses the current every half turn, so the coil spins up to a steady speed in rpm while the voltage it pushes back with cuts the current. Change the field strength, the voltage, the length of wire, the number of turns, and reverse the current or the field to see the force and the turning direction flip.

Follow a population of moths on a tree trunk over many generations. Birds eat the moths that stand out against the bark, the survivors pair up and pass on their colour gene, and the next generation is born. Choose the bark, how many moths are dark at the start, how hard the birds hunt and how well camouflage protects, and watch dark moths take over on dark bark and light moths on pale bark. No single moth ever changes colour: the population changes because the moths that blend in survive and breed more often.

Load a sample of some isotope — adjust the isotope, purity, fuel amount, moderator, and cooling/control rods to see the chain reaction fizzle out, self-sustain, or run away, generation by generation.

Shine light of a chosen frequency onto a metal plate — adjust the frequency and the metal's work function to see whether an electron is ejected at all, and at what speed, exactly as Einstein's photon picture predicts.

Push a thick or thin liquid through a narrow pipe and watch the flow profile form — the fluid at the wall barely moves, the centre races, and halving the radius cuts the flow to a sixteenth.

Dial up starting population, growth rate, carrying capacity, and a harvesting/predation rate and watch a population trace the classic S-shaped curve toward its habitat's limit — or, if harvesting outpaces growth, decline toward extinction. Traced against an uncapped exponential comparison.

Watch rabbits and foxes share a field: rabbits breed on the grass, foxes breed on the rabbits. Because the foxes only multiply after the rabbits have, each peak in the foxes comes after the peak in the rabbits, so the two numbers rise and fall in repeating cycles. Change the food, the foxes' hunting success and how fast they die, or add a drought, disease or the removal of the foxes, to find when the animals live side by side, cycle, or die out.

Dial up acid concentration, temperature, and marble form and watch a gas syringe fill with CO2 — the marble is the limiting reagent, so every run ends at the same volume, only how fast it gets there changes. Traced against a dashed reference run.

Charge or discharge a capacitor through a resistor — adjust the source voltage, resistance, and capacitance to see the exponential voltage/current curves and the circuit's time constant.

Choose your launch bearing, injection speed, flight-path angle, and lead the Moon's own motion — simulate a real Earth-Moon transfer and see if you reach the Moon's sphere of influence. Reaching it refunds your run.

Pick a balanced reaction and the amount of each reactant, and watch particles pair up in the equation's ratio until one reactant runs out. See the limiting reactant, the mass or gas volume of product made, and the excess left behind, and why moles, not grams, decide which runs out first.

Watch a chemical reaction as a hill that the particles have to climb. A profile graph shows the energy of the starting chemicals, the energy of the products and the energy barrier between them, with arrows for the energy given out or taken in. Pick a real reaction, such as burning methane, an acid and an alkali neutralising, calcium carbonate breaking down or ammonium nitrate dissolving, and see whether it warms or cools its surroundings and by how many degrees. Beside it, moving particles show how many have enough energy to get over the barrier at the temperature you choose, and pairs of them react as the run goes on. Switch on a catalyst to lower the barrier without changing the energy given out or taken in, and raise the temperature to see more particles succeed.

Release a hoop, a solid disk, and a solid sphere from rest together — each on its own ramp curve (straight incline, brachistochrone, exponential decay, circular arc, or parabola) — and see how both the SHAPE (moment of inertia) and the PATH decide who reaches the bottom first.

Spin up a solid disc, a hoop, and a solid sphere — all sharing the same mass and radius — under the same applied torque, and watch them visibly pull apart purely because of how each one's mass is spread out from the axle.

Fire a beam of alpha particles at a single heavy nucleus — adjust the alpha energy, target atomic number, particle count, and impact-parameter spread to watch Coulomb repulsion alone deflect each trajectory, most only slightly and a rare few sharply back the way they came.

Pluck a string fixed at both ends into a chosen harmonic — adjust its length, tension, and mass per unit length to see how fast, and in how many nodes and antinodes, it vibrates.

Place an object in front of a converging or diverging lens — watch the three principal rays trace out where the image forms, and see whether it's real or virtual, upright or inverted, magnified or reduced.

Put a planet next to a moon or a star and see how gravity pulls the near side harder than the far side — the stretch that raises two tides a day, locks the Moon's face to us, and can tear a moon apart if it strays too close.

Add strong base to an acid, strong or weak, and trace pH against volume — see the equivalence point's sharp jump and, for a weak acid, the buffer plateau where pH equals pKa at the half-equivalence point.

See how a transformer steps a voltage up or down. Alternating current in the primary coil makes a changing magnetic field in the iron core, and that field induces a voltage in the secondary coil. The voltages follow the turns, so the secondary voltage is the primary voltage times the secondary turns divided by the primary turns, while the current changes the other way and the power stays almost the same. Watch the field in the core swing back and forth and the two voltages on one graph. A second view sends a fixed amount of power down a cable and compares the heat lost with and without stepping the voltage up, which is why power lines use high voltage. Set the voltage, the turns on each coil, the load, the efficiency and the mains frequency. For instance, 12 V across 100 turns into 500 turns gives 60 V.