Hands-on physics demonstrations using everyday objects — designed to spark curiosity and make abstract concepts tangible. Collected over twenty years in classrooms.
01Two Martini GlassesDensity
SetupFill two martini glasses to half their volume — they appear ¾ full. Ask students to predict what will happen when one is emptied into the other.
What happens & whyThe second glass fills exactly to its rim. The conical shape of a martini glass means the upper half holds the same volume as the lower half — a vivid demonstration of how vessel geometry deceives our intuition about volume.
VolumeGeometryEstimation
02Filling or Spilling BottlesDensity
SetupCollect a range of bottles, beakers, and containers of different shapes. Fill one to a marked level and ask students to predict which others it will fill exactly, overfill, or not reach.
What happens & whyResults are often surprising. Tall thin bottles hold far less than squat wide ones of the same apparent height. Leads naturally into a discussion of how packaging exploits this — and why "value size" bottles sometimes contain less than they appear to.
VolumeEstimationConsumer science
03Floating OrangeDensity
SetupPlace an unpeeled orange in a large bowl of water — it floats. Now peel it completely, removing all pith, and return it to the water.
What happens & whyThe peeled orange sinks. The peel is full of air pockets that reduce the orange's average density below 1 g/cm³. Remove them and the denser fruit flesh sinks. A simple and striking demonstration of average density.
Average densityBuoyancyArchimedes
04Floating Various FruitsDensity
SetupFloat a lemon, lime, and apple in a large tank of water. Ask students to predict the outcome before each one goes in.
What happens & whyApples float (average density just below 1). Lemons float — their thick, pithy, air-filled skin keeps average density low. Limes sink — their thinner skin and denser flesh tips the average density just above 1. Great for discussing what "density" actually means for a composite object.
Average densityBuoyancyPrediction
05Floating Egg in SaltwaterDensity
SetupPlace a raw egg in a glass of fresh water — it sinks. Dissolve a large quantity of table salt into the water, a spoonful at a time, and ask students to watch carefully.
What happens & whyAs salt concentration rises, the water's density increases until it exceeds the egg's density (~1.03 g/cm³) and the egg floats. Compare with the Dead Sea (~1.24 g/cm³) where humans float effortlessly, or the Blue Lagoon in Iceland.
Fluid densityBuoyancySolutionsDead Sea
06Coke vs. Diet CokeDensity
SetupTake new, sealed, room-temperature cans of regular Coke and Diet Coke. Lower both gently into a large tank or bucket of cold water.
What happens & whyRegular Coke sinks; Diet Coke floats. A 330 ml can of regular Coke contains about 35 g of sugar — enough to raise the average density of the contents above 1 g/cm³. Artificial sweeteners in Diet Coke are used in much smaller quantities, leaving the density below 1. A memorable density demo that uses objects students know well.
DensitySolutionsBuoyancyEveryday science
07Cartesian DiverDensity
SetupAttach small paperclips to a ketchup or soy sauce sachet until it just barely floats in a 2 L bottle filled to the brim with water. Screw the top on tightly.
What happens & whySqueezing the bottle increases the pressure, which compresses the small air pocket inside the sachet. The sachet's volume decreases while its mass stays the same, so its density increases above 1 g/cm³ and it sinks. Release the pressure — it rises again. Named after René Descartes. Introduces pressure transmission and compressibility.
PressureCompressibilityDensityPascal's law
08Dancing SultanasDensity
SetupPour a glass of cheap, very fizzy lemonade and drop in several sultanas. Watch — this takes a minute or two to develop fully.
What happens & whySultanas sink initially (denser than the liquid). CO₂ bubbles nucleate on the rough surface of the sultana, accumulating until the combined density of sultana + bubbles is less than 1. The sultana rises, the bubbles pop at the surface, and it sinks again. The cycle repeats until the lemonade goes flat. A simple and endlessly watchable demo of buoyancy and nucleation.
BuoyancyNucleationCO₂Average density
09Grapes in LemonadeDensity
SetupDrop one peeled grape and one unpeeled grape into a glass of fizzy lemonade simultaneously.
What happens & whyThe peeled grape stays at the bottom — its smooth surface provides almost no nucleation sites, so no bubbles form to lift it. The unpeeled grape rises and falls repeatedly — its textured skin provides sites where CO₂ bubbles form readily. A neat controlled comparison of nucleation surface effects.
NucleationSurface textureBuoyancyCO₂
10Burning Tea BagThermal
SetupOpen a cylindrical teabag, remove the staple, and shake out the contents. Stand the empty tube upright on a heatproof surface and light the top.
What happens & whyThe bag burns steadily downward. As it nears the base, the remaining ash is so light that the convection current of hot air rising from the flame is enough to lift it — the glowing ash rises dramatically into the air. Demonstrates convection, combustion, and the density of hot gases. Works best in still air.
ConvectionHot gas densityCombustion
11Three CandlesThermal
SetupLight three candles of different heights inside a glass jar or bell jar. Ask students which will go out first when the jar is inverted over them.
What happens & whyMost students predict the tallest candle goes out last (it's furthest from the CO₂ that sinks). In fact it goes out first. Hot CO₂ from combustion is less dense than the surrounding air and rises — it fills from the top. The tallest candle is smothered first. A reliable counterintuitive result that rewards careful thinking about hot vs. cold gas density.
Hot gas densityConvectionCO₂Combustion
12Inertia with a Toy TruckForces
SetupBalance a raw egg on top of a toy truck. Push the truck into a fixed barrier so it stops suddenly.
What happens & whyThe egg continues forward, falls, and breaks spectacularly. The truck experiences a large contact force from the barrier that decelerates it; the egg has no such force applied and continues at its original velocity — Newton's first law. Use a hard-boiled egg for a less messy version, or do it over a tray. An unforgettable demo of inertia.
Newton's 1st lawInertiaMomentum
13Spinning EggsForces
SetupHave one hard-boiled and one raw egg, unlabelled. Spin both on a smooth surface. Briefly touch each to stop it, then immediately release your finger.
What happens & whyThe hard-boiled egg stays stopped. The raw egg restarts spinning after you release it. Inside the raw egg, the fluid yolk and white continue rotating when the shell is momentarily stopped — the fluid's angular momentum is transferred back to the shell via viscous drag, restarting the spin. A useful and elegant way to identify a cooked egg without cracking it.
Rotational inertiaAngular momentumViscosity
14Tablecloth TrickForces
SetupLay a smooth cloth on a table with crockery, a teapot, or other objects placed on it. Pull the cloth sharply downward and away in one fast, smooth motion.
What happens & whyDone correctly, the crockery barely moves. The key is the very short contact time — the friction force between cloth and crockery acts for only a fraction of a second, so the impulse (F × t) is tiny, and the change in momentum of the crockery is negligible. A slower pull gives the friction force longer to act and everything slides off. Classic Newton's first law with a quantitative element.
Newton's 1st lawInertiaImpulseFriction
15Rope on a CylinderForces
SetupTie a 1 kg mass (or tin of beans) to a length of thick string. Wrap the string three or four times around a smooth rolling pin or wine bottle and lift the rod.
What happens & whyA few turns of rope hold the mass without any slipping. Each wrap multiplies the friction force exponentially — this is the capstan equation (T₂/T₁ = e^μθ). With each complete turn, the holding force increases by a factor of e^(2πμ). Used by sailors to hold massive loads with small forces. Leads beautifully into demo #16.
FrictionCapstan equationExponential
16The Falling CupForces
SetupThread about 1 m of string over a pencil. Tie a china cup close to the pencil at one end. Tie a cork to the long end. Hold the string just below horizontal and release.
What happens & whyThe cup falls — but stops before hitting the ground. Conservation of angular momentum causes the cork to spin faster as the string shortens. The increasing speed wraps string around the pencil, and the growing friction slows and stops the cup's descent. The exact stopping point depends on the string length, mass ratio, and friction. A lovely piece of coupled mechanics.
Angular momentumConservationFrictionCoupled systems
17Ball Drop TestForces
SetupHold a table tennis ball and a golf ball at the same height. Drop them simultaneously from about 1 m. Then repeat from 3 m or higher if possible.
What happens & whyFrom 1 m they land together — gravitational acceleration is the same regardless of mass (Galileo). From greater height, air resistance becomes significant. The table tennis ball has a much lower terminal velocity (~9 m/s) than the golf ball (~32 m/s), so from sufficient height the golf ball lands first. Two demonstrations in one: equivalence principle, then real-world air resistance.
Equivalence principleAir resistanceTerminal velocityGalileo
18Coin and Paper DropForces
SetupDrop a coin and a sheet of A4 paper separately. Then hold the paper edge-on alongside the coin. Finally, balance the paper flat on top of the coin and drop both.
What happens & whySeparately, coin lands first. Edge-on, they land together — the paper's cross-section is tiny so air resistance is negligible. Paper on top of coin: the coin falls away first, paper slips off. The coin moves into undisturbed air and falls faster. Three distinct outcomes from one prop, illustrating how air resistance depends on speed, area, and relative motion.
Air resistanceCross-sectionNewton's 2nd law
19Water Jet in Free FallFluids
SetupMake a small hole near the base of a plastic bottle and fill it with water — the jet flows freely. Now throw the bottle upward so it goes into free fall.
What happens & whyThe jet stops completely while the bottle is in free fall. The water only flows because gravity accelerates it faster than the bottle — when both are in free fall together, there is no relative acceleration. The water experiences weightlessness inside the bottle. This is exactly the same reason astronauts float in the ISS — they are in free fall together with the station.
Free fallWeightlessnessFluid pressureOrbital mechanics
20Straw in a Sealed BottleFluids
SetupInsert a vertical straw through a sealed plastic bottle lid. Blow air into the bottle so the increased pressure pushes water up the straw. Hold it sealed. Stand on a chair and jump off (carefully).
What happens & whyAs the bottle goes into free fall, the water shoots up and out of the straw. In free fall, the effective weight of the water column disappears — the compressed air inside now has nothing to support and violently expels the water. A dramatic demonstration of free fall and pressure.
Free fallAir pressureFluid statics
21Groan Tube DropForces
SetupHold a groan tube vertically — it makes its characteristic sound as the internal mechanism rattles. Drop it and catch it again.
What happens & whyThe tube falls silently — the internal rattling mechanism and the tube are both in free fall, so there's no relative motion between them and no sound. When caught, the tube decelerates while the internal mechanism continues momentarily — the groaning restarts. The same physics explains why objects feel weightless in orbit: they are in continuous free fall around the Earth.
Free fallWeightlessnessRelative motion
22Slinky Spring DropForces
SetupHold a slinky at the top, let it hang fully extended, and ask students to watch the bottom carefully. Release the top.
What happens & whyThe bottom of the slinky remains completely stationary until the top arrives and the whole spring collapses onto it. The centre of mass falls at g, but the bottom link is in equilibrium (spring tension balances gravity) and receives no information about the release until the compression wave reaches it. A genuinely strange result — works beautifully in slow-motion video.
Centre of massWave propagationFree fallSpring forces
23Tennis Ball on a BasketballForces
SetupBalance a tennis ball on top of a basketball. Hold them both at shoulder height and drop together onto a hard floor.
What happens & whyThe tennis ball rebounds to approximately 9× the drop height — it appears to shoot violently upward. The basketball bounces first and is travelling upward when it collides with the still-downward-moving tennis ball. In this frame the relative speed is doubled, and since the basketball is much more massive, almost all the kinetic energy is transferred to the tennis ball. See also: Astroblaster toy. Excellent Y12 momentum problem.
Conservation of momentumElastic collisionEnergy transferNewton's cradle
24Two Metre Rules with WeightsForces
SetupClamp a weight to the top of one metre rule and to the centre of another. Hold both at the same angle and release simultaneously.
What happens & whyThe rule with the weight at the centre hits first. When a rod falls freely, its tip accelerates faster than g because the rod rotates about its pivot point. The rate of rotation depends on the moment of inertia — moving mass toward the centre reduces it and increases the angular acceleration, so the tip sweeps faster.
Moment of inertiaRotational dynamicsCentre of mass
25Weighted vs. Unweighted RulesForces
SetupRepeat demo #24 but compare a rule with a weight at its centre against a plain rule with no weight. Drop simultaneously from the same angle.
What happens & whyThey land at exactly the same time. The added mass at the centre increases the gravitational torque and the moment of inertia by the same proportion, so the angular acceleration — and thus the fall time — is unchanged. A precise analogue of the equivalence principle for rotational motion.
Equivalence principleMoment of inertiaRotational dynamics
26Broom Handle on FingersForces
SetupRest a broom handle horizontally on two index fingers — one about ¼ of the way along, the other near the far end. Slowly slide both fingers toward the centre.
What happens & whyBoth fingers always meet at the centre of mass, regardless of starting position. The finger closer to the CoM bears more of the weight and experiences greater normal force — and therefore greater friction — so it moves less easily. The other finger slips first. They alternate in tiny steps, always converging on the CoM. A self-correcting mechanical system.
Centre of massFrictionNormal forceMoments
27Shifting Centre of MassForces
SetupTie a mass (e.g. a heavy bunch of keys) firmly to one end of a broom handle. Repeat demo #26 with your fingers.
What happens & whyFingers now meet at the new centre of mass — shifted toward the weighted end. A clean extension of #26 that makes the concept of centre of mass as a physical location concrete and measurable.
Centre of massMomentsFriction
28Egg Thrown at a SheetForces
SetupTwo people hold a thin vertical sheet (old bed sheet works well), with a trough or folded lip at the bottom. Throw a raw egg hard at the centre of the sheet.
What happens & whyThe egg does not break. The sheet stretches and billows, increasing the stopping distance enormously and spreading the deceleration over a long time. F = Δp/t — if t is large, the peak force on the egg is small. The same principle is used in safety nets, crumple zones, and airbags. Works every time if you hit the sheet cleanly.
ImpulseForce-time graphsSafety engineeringNewton's 2nd law
29Spinning Cream EggForces
SetupSpin a Cadbury's Cream Egg fast on a flat, rough surface. Watch carefully from the side.
What happens & whyAs it spins, the egg rises up onto one end and balances there — similar to a "tippe top". The viscous fondant filling sloshes as the egg spins, shifting the centre of mass. The resulting gyroscopic and frictional torques cause the egg to precess upright. The full analysis is non-trivial; see
IOP analysis ↗. A great discussion starter on gyroscopes.
Gyroscopic motionPrecessionAngular momentumTippe top
30Newspaper and Thin WoodForces
SetupPlace a large sheet of newspaper flat on a table with a thin (3–4 mm) strip of wood underneath it, one end protruding about 20 cm. Strike the protruding end sharply downward with a stick.
What happens & whyThe wood snaps cleanly. Surprisingly large force is needed to accelerate the mass of air above the newspaper — F = ma applies to air too. The paper acts as a seal preventing the air above from rushing in to equalise pressure quickly. The newspaper exerts a large downward force on the wood, which acts as a fulcrum. Quantitative comparison: F = Δmv/t for a large column of air is significant.
Newton's 2nd lawImpulseAir pressureMoments
31Blowing Between Paper StripsFluids
SetupHold two strips of paper vertically, parallel and about 5 cm apart. Blow a steady stream of air between them.
What happens & whyThe strips are drawn together. Faster-moving air between the strips has lower pressure than the still air on the outer faces (Bernoulli's principle). The pressure difference pushes the strips inward. Students often predict the opposite. Note: the full explanation involves streamlines and is more subtle than often taught — good for discussion of model limitations.
BernoulliPressureFluid dynamicsStreamlines
32Newton's Cradle VariationsForces
SetupUse a standard Newton's cradle. Try: (a) placing a piece of metal between two stationary balls, then releasing one; (b) striking one end with a small hammer whose mass exceeds the total mass of all balls.
What happens & why(a) With metal inserted: the behaviour changes as the effective mass of the struck ball increases. (b) Heavy hammer: all balls move — conservation of momentum requires it when the impacting mass exceeds the total struck mass. These variations break students' expectation that Newton's cradles always "just work" and reveal the underlying conservation laws.
Conservation of momentumElastic collisionNewton's 3rd law
33Newton's Cradle — Bowling BallsForces
SetupSet up three 6 lb bowling balls in contact on a smooth floor. Roll an 8 lb ball into them.
What happens & whyOne 6 lb ball exits the far end while the 8 lb ball slows but doesn't stop — momentum and energy are conserved but not by the simple "one-in, one-out" rule of equal-mass cradles. Change the incoming mass to 12 lb and two balls exit. The demonstration makes the role of mass in momentum conservation unavoidable.
Conservation of momentumElastic collisionMass dependence
34Waddling Animal on a SlopeForces
SetupUse a commercially available waddling toy that walks down a ramp. Add a small mass to different positions along its body and release from the same point.
What happens & whyPlacement matters: adding mass above the centre of mass raises it and increases the tendency to topple, speeding the waddle. Adding mass low or at the pivot has less effect. This provides a tangible entry point into stability, centre of mass height, and potential energy in mechanical systems.
Centre of massStabilityPotential energy
35Wine Glass and CorkFluids
SetupPlace half a cork inside a wine glass so it rests on the base. Blow a sharp, steady stream of air across the rim of the glass.
What happens & whyThe cork shoots upward out of the glass. Fast-moving air across the rim reduces pressure above the cork (Bernoulli effect), and the higher-pressure air below pushes it up. A Venturi / Bernoulli demonstration that works reliably and surprises students who expect the cork to blow sideways.
BernoulliVenturiPressureFluid dynamics
36Soap-Powered BoatFluids
SetupCut a small boat shape from a lolly stick or thin card, with a small notch at the stern. Float it on perfectly still water in a large dish. Drop a tiny grain of soap powder into the notch.
What happens & whyThe boat moves forward. Soap molecules reduce surface tension wherever they spread — but they spread backwards first, pulling the boat forward by the unequal surface tension forces. Works only once per dish of water (the soap spreads to equilibrium). A demonstration of surface tension as a real, directional force.
Surface tensionMarangoni effectSurfactants
37Cocktail Sticks and SoapFluids
SetupArrange 5–6 cocktail sticks in a star pattern on still water. Touch the centre with a small piece of dry absorbent paper, then (on a fresh surface) touch the centre with a tiny drop of soap.
What happens & whyPaper: sticks move inward as a small amount of water is absorbed, locally increasing surface tension. Soap: sticks shoot outward as the soap dramatically reduces surface tension at the centre — the higher surface tension at the periphery pulls them out. Two distinct surface tension effects demonstrated in sequence.
Surface tensionMarangoni effectSurfactants
38Knees Against a WallForces
SetupStand with both heels touching a wall. Attempt to bend your knees and crouch down without leaning forward or taking your heels off the floor.
What happens & whyIt's impossible — you immediately fall forward. Normally when you bend your knees, your centre of gravity moves backward and your torso leans forward to compensate. The wall prevents the necessary rearward shift, so your centre of gravity moves outside your base of support and you topple. A bodily demonstration of CoG and stability.
Centre of gravityStabilityBase of support
39Breaking a Suspended TinForces
SetupTie a piece of thread around a tin of beans and suspend it from a support. Attach a second piece of thread below the tin. Pull the lower thread — first slowly, then (with a fresh setup) very sharply.
What happens & whySlow pull: the upper thread breaks (it bears the tin's weight plus the pulling force). Sharp pull: the lower thread breaks (the force is applied faster than the tin's inertia can be overcome — the tin barely moves, so the tension in the lower thread briefly exceeds that in the upper). A definitive demonstration of the role of time in F = Δp/t.
ImpulseInertiaF = Δp/tNewton's 2nd law
40Candle at Both EndsForces
SetupPush a long needle through the centre of a long candle. Shave the wax from both ends to expose wicks. Balance the needle horizontally on the rims of two glasses. Light both wicks.
What happens & whyThe candle rocks like a seesaw and keeps going. As one end burns and drips wax, it becomes lighter, that end rises, the other dips and burns faster, then that end lightens — the system oscillates indefinitely. A self-sustaining moments demonstration with no input of energy beyond the candles themselves.
MomentsTorqueEquilibriumOscillation
41Mirror WritingOptics
SetupWrite the names TOM, DICK, and HARRY in clear capital letters and hold the paper horizontally beneath a vertical mirror.
What happens & whyOnly DICK appears completely unchanged in the reflection. TOM and HARRY are reversed because their letters lack vertical symmetry. DICK is made entirely of letters with left-right symmetry (D, I, C, K) so the mirror reflection is identical to the original. Triggers great discussion on what mirrors actually do — they reverse front-to-back, not left-to-right. See also:
IFLScience ↗
ReflectionSymmetryPlane mirrorsLateral inversion
42The Big CircuitEM
SetupRun wires from a power supply at the front of the room to the back wall and back to a bulb. The total wire length should be as long as practically possible — 20–30 m works well.
What happens & whyThe bulb lights instantly when the switch is closed. Students expect a delay — the "electricity travelling through the wire" mental model suggests it should take time. In fact, the electromagnetic field propagates at close to c (~3×10⁸ m/s). At 30 m, the delay is 0.1 μs — completely imperceptible. Extend the discussion: signals around the Earth (~130 ms), speed-of-light constraints in high-frequency trading, and latency in global networks.
Speed of lightEM field propagationElectric circuitsSignal latency
43Whacking a Suspended BroomForces
SetupSuspend a broom handle horizontally by two thin threads attached to its ends. Strike the centre of the handle sharply with another broom handle. ⚠️ Wear eye protection; warn students to stand clear.
What happens & whyStruck hard enough, the handle snaps — but the threads survive intact. The inertia of the handle resists the sudden force; the impulse is absorbed by the wood before it can be transmitted to the ends and thence to the threads. The threads experience almost no force. Classic inertia demonstration — the same principle as tablecloth trick, but more dramatic.
InertiaImpulseNewton's 1st law
44Ice on Different SurfacesThermal
SetupPlace identical ice cubes simultaneously on: (a) a plastic chopping board, and (b) an upturned metal frying pan. Both feel cold to the touch — ask students to predict which will melt faster.
What happens & whyThe ice on metal melts dramatically faster — often within a minute. Both surfaces are at room temperature, but metal has a thermal conductivity ~1000× higher than plastic. It conducts heat from the room to the ice far more rapidly. "Cold to the touch" is not temperature — it is rate of heat transfer. One of the most reliable misconception-busters in the collection.
Thermal conductivityRate of heat transferMisconceptionsTemperature vs. heat
45Magnet on Japanese YenEM
SetupStack a few Japanese yen coins on a flat surface. Place a strong neodymium magnet on top. Lift the magnet quickly upward.
What happens & whyThe coins lift with the magnet momentarily before falling away. Yen coins are made of an aluminium alloy — non-magnetic. But moving the magnet away rapidly induces eddy currents in the coins (Lenz's law). These currents create a magnetic field opposing the change, temporarily attracting the coins to the magnet. The effect disappears as the currents decay. A more striking Lenz's law demo than the copper pipe.
Lenz's lawEddy currentsElectromagnetic inductionFaraday's law
46Suspended Yen Near MagnetEM
SetupStick a thread to a yen coin and suspend it so it hangs freely. Slowly bring a strong neodymium magnet toward the coin.
What happens & whyThe coin swings away from the approaching magnet. As the magnet approaches, eddy currents induced in the aluminium alloy create a repulsive magnetic field (Lenz's law — the induced effect opposes the cause). The coin is repelled. A non-magnetic coin behaving magnetically: reliably astonishing to students who think only iron is affected by magnets.
Lenz's lawEddy currentsElectromagnetic inductionNon-ferrous metals
47Floating a Yen on WaterFluids
SetupLower a yen coin very carefully onto still water using a small piece of tissue paper — let the tissue absorb water and sink, leaving the coin floating. Add drops of water to the top of the coin one at a time.
What happens & whyThe coin floats, supported by surface tension acting along its rim. You can add several drops before it sinks. Then add a single drop of washing-up liquid — the coin sinks immediately. Surface tension provides a measurable upward force; soap (a surfactant) disrupts the hydrogen-bonded water surface and it fails. The contact angle between water and the coin edge is key.
Surface tensionSurfactantsContact angleWater structure
48Two Test TubesDensity
SetupFind two test tubes where the smaller just fits loosely inside the larger. Fill the large tube with water, place the small tube inside (open end down) so it floats, then invert the whole assembly with your thumb over the mouth.
What happens & whyWhen inverted, the small tube rises to the bottom (now the top). The air trapped in the small tube makes its average density less than water, so it floats upward — which, when inverted, is the closed end. A neat Archimedes demonstration with a counterintuitive twist.
BuoyancyAverage densityArchimedes
49Glass Rod vs. Foil Rod Near MagnetEM
SetupSuspend a short (~3 cm) glass rod on a thread, and separately a similarly sized rolled aluminium foil rod. Hold a strong neodymium magnet near each in turn.
What happens & whyThe glass rod orients perpendicular to the field — glass is diamagnetic and is weakly repelled, aligning across field lines. The foil rod aligns parallel to the field — aluminium is paramagnetic and is weakly attracted, aligning along field lines. Most students only know ferromagnetism; this demonstrates that all materials interact with magnetic fields, just very weakly.
DiamagnetismParamagnetismMagnetic properties of materials
50Magnet Rolling on a SlopeEM
SetupSet up a very gentle slope aligned precisely north–south. Release a small but strong cylindrical magnet to roll freely.
What happens & whyThe magnet follows a curved arc rather than a straight path. As it rolls, its magnetic moment interacts with the Earth's magnetic field — the field exerts a torque that causes the magnet to veer. A rare and surprising demonstration of the Earth's field doing mechanical work on a rolling object.
Earth's magnetic fieldMagnetic torqueGyroscopic effects
51Eclipse of Mars AfterimageOptics
SetupProject or display a bright red disc on a screen. Ask students to stare at its centre without blinking for 20–30 seconds. Then show a plain white screen or ask them to look at a white wall.
What happens & whyA cyan disc appears — the complementary colour. The red-sensitive cones in the retina become fatigued (bleached) by prolonged stimulation. When you look away, those cones respond less strongly to the white light, so the eye perceives the colour that would result from green and blue cones alone — cyan. A reliable, striking entry into colour perception and the biology of vision.
AfterimageColour visionCone cellsComplementary colours
52Skewers Through a Water BagFluids
SetupFill a resealable plastic bag with water. Carefully push a sharpened barbeque skewer through one side and out the other in one smooth motion.
What happens & whyNo water leaks. The polymer chains of the plastic bag are long and flexible — the skewer parts them and they seal around it under pressure from the water inside. This is the same property that makes self-sealing tyres and surgical gloves work. Works equally well with a sharpened pencil. Dramatic done over a student's head (having practised first).
Polymer propertiesSurface sealingMaterials science
53Elastic Band TemperatureThermal
SetupHold an elastic band to your lip (sensitive to temperature). Note its temperature. Stretch it rapidly and fully, then immediately hold it to your lip again. Wait 5 seconds, then let it snap back and immediately hold it to your lip.
What happens & whyStretching warms it; retracting cools it. Rubber consists of long polymer chains that become more ordered (lower entropy) when stretched. To conserve total entropy, the thermal energy increases — warming. Releasing restores disorder, absorbing thermal energy — cooling. A tangible, personal introduction to entropy and the thermodynamics of rubber elasticity.
EntropyRubber elasticityPolymer thermodynamicsThermodynamics
54Two Balloons ConnectedFluids
SetupInflate one balloon large and one balloon just slightly. Connect them via a tube with clips, then open the clips simultaneously.
What happens & whyThe small balloon deflates into the large one. Counterintuitive — students expect equalisation. Internal pressure in a rubber balloon is P = P₀ + 4T/r, where T is surface tension and r is radius. A smaller balloon has a smaller radius and therefore higher internal pressure. When connected, the pressure gradient drives air from small to large. Only equalises if both are in the flat or very large regime.
PressureSurface tensionLaplace pressureRubber balloons
55Inflating Two Balloons TogetherFluids
SetupConnect two uninflated balloons to the arms of a Y-piece (or T-piece) of tubing. Blow into the stem and try to inflate both simultaneously.
What happens & whyOne balloon inflates much more than the other. Whichever balloon starts slightly larger has a slightly lower internal pressure (same P = 4T/r argument as #54), so air preferentially flows into it, making it larger still. A positive feedback loop — the system is unstable. The other balloon stays small. Works every time, regardless of effort.
PressureLaplace pressurePositive feedbackInstability
56Floating Beaker in a BottleDensity
SetupFloat a small beaker containing a heavy object (e.g. a stone) in a 2 L bottle of water. Note the water level. Remove the stone from the beaker and place it directly in the bottle water. Note the level again.
What happens & whyWhen the stone is in the floating beaker, it displaces water equal to its weight. When placed directly in the bottle, it sinks and displaces only its own volume (much less, since it's denser than water). The water level falls. Classic Archimedes — the difference between displacement by weight and displacement by volume.
ArchimedesDisplacementBuoyancyFloating vs. sinking
57Sealed Jar From a BoatDensity
SetupFloat a toy boat carrying a sealed, empty glass jar in a bowl of water. Note the water level carefully. Remove the jar from the boat and float it separately alongside.
What happens & whyNo change in water level. The jar is sealed and floats — in both cases the total weight of boat + jar is the same, and by Archimedes the total displaced volume (= total weight / ρg) is unchanged. Compare with demo #56 where an object sinks: a sealed jar remains a floating object regardless of whether it's on the boat or beside it.
ArchimedesDisplacementBuoyancy
58Paper Clip on Thread Near a MagnetEM
SetupTie a paper clip to a long thread. Lower it slowly toward a strong magnet, then raise it again.
What happens & whyThe paper clip rotates in one direction as it descends, the opposite direction as it rises. The magnetic field lines curve around the magnet, and the paper clip (a small magnetic dipole) aligns with the local field direction. As it traverses different regions of the field geometry, the torque on it reverses. A low-key but precise demonstration of field geometry.
Magnetic field linesMagnetic dipoleField geometry
59Measuring Cylinders of Different DiametersDensity
SetupTake two measuring cylinders where one has exactly twice the diameter of the other. Pour a fixed volume from the larger into the smaller.
What happens & whyThe liquid stands four times higher in the narrower cylinder. Cross-sectional area scales as r², so halving the diameter quarters the area. For the same volume, the height must be four times greater. A simple but powerful illustration of why ratio reasoning with areas and volumes requires squaring and cubing — not just proportional scaling.
VolumeArea scalingRatio reasoningGeometry
60Balancing Tin Cans on a Metre RuleDensity
SetupHang two identical tins of water at equal distances from the centre of a metre rule so it balances horizontally. Lower one finger into one of the tins — without touching the sides or bottom.
What happens & whyThe rule tips toward the tin your finger is in. Your finger displaces water — by Archimedes, the water exerts an upward buoyancy force on your finger and an equal downward reaction on the tin. The tin effectively gets heavier, tipping the balance. A precise and elegant Archimedes demonstration that works on the first attempt every time.
ArchimedesBuoyancyNewton's 3rd lawMoments
61Dropping Paper and BookForces
SetupDrop a small piece of paper and a large book separately. Then try: (a) paper below the falling book, (b) paper resting flat on top of the book.
What happens & why(a) Paper below book: paper falls in the partial vacuum behind the book and lands simultaneously. (b) Paper on top of book: the book pushes through undisturbed air; the paper is carried on top and also falls simultaneously. In both cases the paper avoids encountering the air head-on. A simple sequence revealing the role of relative motion in air resistance.
Air resistanceDragRelative motionFree fall
62Ball in a Whirled BucketForces
SetupPartly fill a clear polythene bucket with water and float a small ball on the surface. Whirl the bucket smoothly in a vertical circle at moderate speed.
What happens & whyThe ball stays on the surface throughout the rotation — including when the bucket is inverted. In the rotating frame, the centrifugal effect pushes everything outward (toward the bottom of the bucket, which is always the "outer" wall). The ball floats on the surface relative to the bucket because it is less dense than water — and the effective gravitational field always points "outward" from the axis of rotation.
Circular motionCentripetal forceRotating framesBuoyancy
63Floating Orange in a Beaker on ScalesDensity
SetupPlace a beaker of water on a balance and tare it to zero. Place an orange beside (but not in) the beaker. Note the reading. Now place the orange in the water so it floats.
What happens & whyThe reading increases by exactly the weight of the orange. The orange exerts a buoyancy force on the water equal to its own weight (since it floats). By Newton's 3rd law, the water exerts an equal downward force on the beaker — adding exactly the orange's weight to the balance reading. A precise and elegant result: the scale reads total weight regardless of floating.
ArchimedesNewton's 3rd lawBuoyancyScales and balance
64Elastic Collision — Bowling BallsForces
SetupSet up two bowling balls on a smooth, level floor or rails. Place one stationary. Roll another into it directly.
What happens & whyFor equal masses, the rolling ball stops and the stationary ball moves away — a classic 1D elastic collision. Interestingly, the struck ball then begins to spin and rolls back toward the first — rotational kinetic energy was not conserved in the collision, and the friction force from the floor causes the non-spinning ball to accelerate rotationally and then reverse. The full treatment is excellent Y13 mechanics.
Elastic collisionConservation of momentumRotational dynamicsFriction
65Magnet Falling in Copper PipeEM
SetupDrop a strong neodymium magnet down a vertical copper pipe. Drop a non-magnetic object of the same size and approximate mass down the same pipe for comparison.
What happens & whyThe magnet falls in slow motion — it takes many seconds to descend what should take a fraction of a second. The moving magnetic field induces eddy currents in the copper pipe. By Lenz's law, these currents create a magnetic field opposing the magnet's motion. The braking force is proportional to velocity and eventually balances gravity, giving a constant terminal velocity. A viscous fluid analogy for electromagnetic braking.
Lenz's lawEddy currentsElectromagnetic brakingFaraday's law
66Candle in WaterThermal
SetupPlace a candle in a beaker and fill with water until it reaches precisely the level of the top of the candle, without wetting the wick. Light the candle.
What happens & whyThe candle burns all the way down into the water, forming a thin wax cylinder that keeps the water out. The wax melts from the inside but the outer layer in contact with the cool water solidifies immediately, building a self-reinforcing wall. The candle can burn below the water line indefinitely. A surprising demonstration of thermal conduction, phase change, and material properties.
Thermal conductionPhase changeMelting pointCombustion
67Vanishing Test TubesOptics
SetupPlace a small test tube inside a larger beaker. Fill the beaker with baby oil (mineral oil) until the small tube is fully submerged.
What happens & whyThe inner test tube disappears completely. Glass and baby oil have almost identical refractive indices (~1.47). Light passes from oil into glass without bending at the interface — there is no refraction and no reflection to make the glass visible. We see glass because it refracts light differently from its surroundings. Remove that difference and it vanishes. As a performance piece: lower a "broken" test tube and retrieve an intact one — the oil conceals the swap.
Refractive indexRefractionReflectionOptics
68Balancing a Coin on a BanknoteForces
SetupChallenge students to balance a coin on the edge of a flat banknote without bending or folding it. Then demonstrate: gently bow the note into a slight curve (like a very gentle arch), balance the coin on the edge at the midpoint, then slowly, carefully straighten the note back to flat.
What happens & whyThe coin balances on the flat note. When curved, the coin's centre of gravity sits directly above the contact point and any small perturbation causes a restoring force — stable equilibrium. Straightening the note while the coin is balanced preserves this equilibrium. Similar to balancing a broomstick: the contact point is below the CoG but with the right geometry, small tilts self-correct.
Centre of gravityStable equilibriumRestoring force
69The Free-Fall ParadoxForces
SetupAttach a cup to the end of a stick with a ball resting in a holder above it, positioned so that if the ball fell straight down it would miss the cup. When the apparatus is released to fall freely, the ball lands in the cup every time.
What happens & whyIn free fall, both ball and cup accelerate at g. The ball's horizontal position relative to the cup doesn't change — it falls exactly as far as the cup does. The geometry is arranged so that this relative motion guides the ball in. It works at any drop height. A beautiful application of the principle of superposition of motions, and a cousin of the "monkey and hunter" problem in projectile physics.
Free fallProjectile motionSuperpositionEquivalence principle
No demonstrations match.
69 demonstrations · collected over twenty years · sources include David Featonby, Keith Gibbs, and the Caltech Physics Demonstration Archive.