There are many science fair experiment ideas online. The following may be repetitions.
A windmill connected to an electric generator, which is connected to a lightbulb (small dimmable is best). Blow on the windmill to turn the light on. A separate generator similar to the one attached to the windmill, which can be cranked by hand to turn on the lightbulb. An electric fan that can blow on the windmill, with power consumption at the fan and power production at the windmill measured and displayed. This explains efficiency losses in power generation.
Pressure of light. A piece of paper attached vertically on a platform that can rotate at low friction. On one side of the rotation axis, the paper is painted black, on the other, it is white. Reversed on the opposite side of the paper. Transparent dome over the setup to prevent air currents interfering. Shining a light on the paper makes it rotate, because the pressure on one side is greater. The rotating platform can be a polystyrene disk floating in water.
Pulleys and gear ratios. A bicycle with gears, rear wheel removed, but axle in place. Rope attached to axle, weight to rope. Rotating the pedals lifts the weight. Different gears require different number of rotations to lift the weight to a given height, but the more rotations needed, the less force needed for the rotation. Explain why low gears on a bike should be used when starting and on uphills, but high gears on downhills.
Moving pulleys: the distance the rope has to be pulled becomes longer, but the force required to pull becomes smaller to lift a given weight a given distance.
Friction in braking. Several bicycle wheels with brakes attached. Some rims are dry, some wet, some oiled. Feel the braking force required to stop each. To provide the force that the brakes must counter, a weight can be attached to each wheel. Rotate the weight away from the lowest point of the wheel and try to use the brakes to prevent the weight from sinking to the bottom again.
Friction in accelerating. Old bicycle. Rotate the pedals to accelerate the rear wheel to a given speed (measured with a bicycle speedometer) when the chain is dry, or oiled, or sand is poured on the chain. Feel the different difficulty depending on the condition of the chain. Several bikes is better, so the chains in different condition can be compared.
Friction and heat: a fire drill. Rotate a sharp stick in a hole in a piece of wood – the hole blackens and starts to smoke.
Ball bearings. Stack two blocks, put weight on the top one, try to rotate the bottom one. Now the same blocks with two ball bearings, one between the ground and the bottom bearing and the other between the two blocks. Much easier to rotate the bottom block. Explain how rolling friction is smaller than dragging friction. Friction proportional to pressure. Friction related to surface area.
Tire pressure and friction. Bicycle wheels (preferably with identical rims, hubs and spokes) with different width tires on them. Measure the friction of the tires by the force required to rotate them at a given speed on some surface, with the tire bearing weight. A stationary bicycle trainer can be used. Which tire width gives the lowest friction? Research shows that 22-23 mm tires have the lowest friction under the weight of an adult cyclist. Deflate the tire, measure the friction. Inflate, measure the friction. Which inflation pressure gives the lowest friction? Research shows that it is not the maximal pressure, unless the surface on which the wheel rotates is very smooth.
Mining and ores. Different rock samples of various ores. Panning for “gold”: try to find a shiny grain hidden in a quantity of mud or sand.
Solvents. Pebbles or small toys mixed in sugar paste or syrup, which is then dried into blocks. Use water to dissolve the sugar and discover what is inside the blocks.
Leidenfrost effect. Air hockey table with air being pumped under the hockey puck. Compare to droplets of liquid nitrogen rolling on a warm surface. Compare to pancakes riding on the steam bubbles under them on a hot pan.
Bridge of spaghetti. Dry spaghetti can be attached to each other with small balls of dough (flour mixed with water). Then bake the spaghetti-and-dough construction to harden the dough. Check how much weight the bridge can carry. Compare the breaking weight for a bridge (triangularly connected spaghetti that look like high-voltage power line towers) to the breaking weight for a bunch of horizontal spaghetti. The bridge can also be made by glueing matches or toothpicks.
Detergents. Lightly grease some cloth that normally absorbs water. Put water on top of that cloth – droplets form and nothing leaks through. Put water in a cloth bag and show that it does not leak. Compare to ungreased cloth that lets water through. Add detergent to the water on the greasy cloth. With the right coarseness of cloth, amount of grease and detergent, the water should start leaking through.
A burner under a thin paper box filled with water. The paper does not burn and does not become soggy, so the water does not leak through.
Absorption and radiation. Heat lamp shining on a black and white rock. Which becomes warm first? Now heat the rocks by contact, e.g. in warm water. Which rock cools down first? Infrared laser thermometer may help confirm temperatures. Or an ordinary thermometer inserted in a hole drilled in the rock.
Hot air rises. A nonflammable parachute rises above a candle. The parachute could be of thin tinfoil. It should have a light weight attached below the canopy to keep it upright. A tinfoil pinwheel can be held above the candle – it starts to rotate in the updraft of hot air.
3D printing by hand. Mud dripped from a hand onto sand or another surface that lets water through easily. The water soaks out of the droplets of mud that hit the surface, leaving a series of solid bumps of mud. Drop another droplet of mud on top of the bumps – water soaks out again, the bumps are now higher. Use this technique to build walls, castles etc.
Internal combustion engine. A rotating shaft with two pistons attached (can be made of wood or some other cheap, light, strong enough material). A balloon under each piston. If two people rapidly inflate and let deflate the balloons in the right sequence, then the shaft starts to rotate. Also a great party game – which couple gets their motor running fastest? The balloons can also be inflated by a hand pump, but fast and well-timed deflation is then a problem. One end of a long snakelike balloon can be under the piston, the other end squeezed and released by hand. The right sequence of squeeze and release can make the shaft rotate. One person can hold two long balloons, so can rotate the engine alone. A single piston is enough to rotate a wheel if attached to the rim – this is the arrangement in some old steam locomotives and foot-pedalled Singer sewing machines.
Steam power. Electric kettle boiling water, with a small windmill above the spout. The steam rising from the kettle rotates the windmill. Measure the power generated by the windmill and compare to the electricity required to heat the kettle.
Electric motor. Hand generator produces electricity, which is led by wires to an electric motor, which starts to rotate. Or the electricity is led to a coil with a permanent magnet inside. The magnet starts to move when the hand generator is cranked. With the right speed of cranking, an alternating current can make the magnet rotate.
Water mill. Pour the water at the top of a halfpipe. A waterwheel in the halfpipe starts to rotate in the flow. The rotation can drive a small generator which lights up a tiny lightbulb.
Archimedes’ screw. A helix in a pipe that is slightly tilted from horizontal can be rotated to pump water up the pipe.
Connected vessels. Two cups of water at different heights linked by a bent drinking straw. Suck the bottom of the straw to get water over the hump in the straw. Then water will start to flow from the top cup to the bottom, initially going uphill in the straw.
A bit of charcoal dropped in a vial of pure oxygen spontaneously catches fire. The oxygen can be generated at the bottom of the vial using hydrogen peroxide and a drop of blood. Steel wire in a pure oxygen environment rusts rapidly enough to be observed in a single experiment.
Bimetal thermostat. Two strips of different metals glued, soldered or riveted together at the ends. Heat the joined strip with a hair dryer – it bends to one side. The bent strip can touch a contact and switch something on or off, for example the hair dryer. A feedback loop can be constructed: if the joined metal strip gets cold enough, then it switches on the hair dryer, which heats the strip, which then switches off the hair dryer.
Tracks vs wheels. Tracked and wheeled model vehicles driving in a sandbox and on a smooth surface. The vehicles are the same weight, have the same motor and battery. Compare performance going up a sandhill vs racing on a smooth surface.
A spinning top to illustrate gyroscopes and self-stabilising.
Degrees of freedom of movement. Mechanical devices that illustrate the 3 translation and 3 rotation possibilities by allowing some of them, but not others. For example the serial gimbal, 3-axis gimbal, origami for thick materials (Science 24 Jul 2015: Vol. 349, Issue 6246, pp. 396-400, DOI: 10.1126/science.aab2870), bicycle front fork with shock absorbers, shopping trolley wheel or office chair wheel. For fun, rotate a person on an office chair a few dozen turns, then ask them to walk in a straight line.
Origami: Miura fold, hexaflexagon. Cutting and gluing mathematical objects, e.g. a Mobius strip, a Klein bottle. Math drawing with compass and ruler, e.g. a flower made from 7 interlinked circles of the same diameter. Sierpinsky carpet. Inscribing circles in squares and vice versa.
Boil amaranth in a transparent vessel. The grains dance on the bottom, then form columns, then a goo with large breaking and splattering bubbles.
Water volcano. A small bottle of warm coloured water is dropped in a large transparent vessel containing clear cold water. The coloured water will rise to the top and spread out, like a volcanic ash plume rises in the atmosphere and spreads in a layer.
Twinkling stars and heat mirages. A transparent rectangular vessel of water with points of light behind it. Look through the water: the points do not move. Now heat the water from the bottom: the points of light start to wobble, because warm water currents are rising up and bending the light passing through them.
Refractive index measurement. Water and cooking oil in transparent vessels. One person puts a straight stick at an angle into the liquid – the stick seems to bend at the immersion point. The person points out where they perceive the bottom end of the stick to be. Another person at the side of the vessel measures the angle between the perceived and actual sticks. The difference in angles is different for water than for oil.
Muscles and joints. Ask a person to relax their hand on a table, palm up. Press on the inside forearm and pull it toward the elbow – the 3 smallest fingers bend. Pull the palm towards the wrist – the fingers bend. An excavator’s scoop is moved by pistons like human limbs are moved by muscles – by pulling on the joint on one side or another.
Centrifuge to separate liquids, or solids from a liquid. The low-cost whirligig centrifuge (paperfuge) is described in Nature Biomedical Engineering 1, Article number: 0009 (2017) doi:10.1038/s41551-016-0009 and http://www.nature.com/news/spinning-toy-reinvented-as-low-tech-centrifuge-1.21273 Animal blood or some mixture of liquids and solid flakes can be separated into component liquids and solids by spinning it.
Solar-powered distillation. The 1 square metre device made of polystyrene, paper and charcoal distils about 1 litre of water per hour, as described in http://www.sciencemag.org/news/2017/02/sunlight-powered-purifier-could-clean-water-impoverished Explain capillary action, evaporation and condensation, black material absorbing heat radiation faster.
Passive cooling. Glass beads 8 micrometres in diameter embedded in a polymethylpentene film backed with a thin silver film. The film conducts heat from the surface it sits on and radiates it away in in the other direction. http://www.sciencemag.org/news/2017/02/cheap-plastic-film-cools-whatever-it-touches-10-c
Oil- and water-repellent coating of glass. Cannot be prepared on the spot, but previously fabricated coatings can be demonstrated. Deposit candle soot on glass, then coat with a 25 nanometre silica film by putting the glass in a desiccator with open vessels of tetraethoxysilane and ammonia for 24 hours. Heat to 600C in air for 2 hours. Put in a desiccator with open beaker of semifluorinated silane for 3 hours. Xu Deng et al. “Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating” http://science.sciencemag.org/content/sci/335/6064/67.full.pdf?sid=ca40c018-0715-4d3b-8e36-58c329dea347
Electricity and water analogy. Pouring water down an inclined halfpipe to rotate a water wheel under the bottom of the halfpipe. The water wheel is connected to a winch that can lift a small weight. The height or angle of the halfpipe and the quantity of water can be changed, which lifts the weight at different speeds or can lift a larger weight. The height of the halfpipe is analogous to the potential difference (voltage) and the quantity of water per second to the current (amperage, electrons per second). The water wheel can also be connected to a generator that powers a small dimmable lightbulb. The brightness of the bulb changes with the speed of the water wheel.
Plastic beads in water poured down a halfpipe can demonstrate the measurement of flow per unit of time: the number of beads passing a given point per ten seconds for example.
Ship shape. Make model boat hulls out of some easily worked material, e.g. putty, model clay (hull must be thin to float), cut styrofoam, cardboard taped together and covered with cling film. The styrofoam needs extra weight, for example a piece of sheetmetal pushed through it in the middle to make a keel. The cardboard boat can carry some gravel as ballast. Then each boat gets a motor of equal power, e.g. a propeller powered by a wind-up spring from some toy. Weigh the boats and add ballast as needed to equalise weights. Race the boats to see which hull shape is fastest. To keep the racing boats straight, put them in parallel halfpipes of water, or create swimming pool lanes in a large tub by parallel strings drawn taut on the surface of the water.
Priming the pump. An interesting mechanical device is a piston pump for getting water out of the ground. A piston moves up and down in a vertical pipe open at the top. There is a hole in the side of the pipe from which water can come out. The piston passes above and below the hole as it moves. A dry piston doesn’t pump water up, but pouring some water on the piston (priming the pump) creates a temporary airtight seal around the piston, so pulling the piston up creates a slight vacuum under it, which draws the water up. The water flows out of the hole in the side of the pipe.
Spinning methods and yarn properties. Threads or wires can be spun into yarn by cone spinning, Fermat or dual-Archimedean spinning. With enough twist, the yarn starts to coil like a telephone cord. Folding twisted yarn in two makes the two halves twist around each other – this is how rope is made. Twisted but uncoiled yarn can be coiled in the opposite direction to the twist. Removing the core around which the coiling took place lets the twist and coil cancel – parallel threads result.
Ricequakes as a model of rockfill dam collapse: put puffed rice in a transparent vertical vessel under pressure and let water in from the bottom. The rice starts compacting in sudden collapses, not gradually. Itai Einav and François Guillard “Tracking time with ricequakes in partially soaked brittle porous media” http://advances.sciencemag.org/content/4/10/eaat6961
Capturing water vapour in water beads flowing down vertical cotton threads while the humid air flows up along the threads. The goal is desalination and water purification in general. A Sadeghpour et al 2019 https://advances.sciencemag.org/content/5/4/eaav7662
Soft pneumatic oscillator driven by constant pressure: three cylinders connected a ring, each containing a bistable hemispherical diaphragm. The diaphragm popping to the other side buckles a tube, choking airflow to the next cylinder, which allows its diaphragm to pop back, straightening a tube and allowing airflow. Preston et al. 2019 https://robotics.sciencemag.org/content/4/31/eaaw5496