Search the Web for "perpetual motion machine" and you'll discover intriguing devices that appear to move forever without energy input. Leonardo da Vinci, after attempting to build perpetual machines, eventually gave up and called the task "impossible," and modern scientists agree. Yet that doesn't mean you can't create a school experiment that mimics a perpetually moving machine and illustrates important scientific principles.
The Mysterious Thirsty Toy Bird
A popular drinking bird toy simulates perpetual motion by appearing to drink repeatedly from a water glass. You can make your own version of a drinking bird toy that doesn't require water to operate. Obtain one of these toys and soak its head in hot water to loosen the glue. Scrape away the head's fuzzy coating and hat and then paint the bird's top half silver. Paint the bottom half black, then place the bird back on its stand and place it in the sunlight. As solar energy strikes the bird, it dips its head as if drinking, then returns to an upright position. You may need to adjust the toy's balance point if the bird fails to dip or falls over. You can do that by moving the center metal pivot up or down.
The Science Behind the Bird
Although the bird moves with no intervention, it's not a perpetual motion machine. On the original toy, water evaporates from the head, causing its temperature to drop. Enclosed vapor in the glass tube condenses, resulting in a pressure difference between the bird's top and bottom. Liquid then flows from the base to the head causing the top-heavy toy to dip down. After the pressure between the head and base equalize, the bird moves back into a vertical position and the cycle repeats until no more water is present. When you make your own version of the bird using paint, sunlight instead of evaporating water creates a heat differential between the bird's top and bottom.
The Self-Moving Wheel
If wheels revolved automatically, vehicles could drive forever without fuel. That can't happen, but you can make a wheel appear to revolve on its own by removing spokes from a bicycle wheel and replacing them with rubber bands. Mount the wheel vertically on a fixed axle and spin it a few times to ensure that it revolves freely. Place two heat lamps on one side of the wheel and the wheel will rotate. Heat causes the rubber bands on the side facing the lamps to contract. The contraction shifts the wheel's center of gravity, allowing gravity to turn it. Placing the project inside a glass display can help convince spectators that the lights are simply there to illuminate the wheel.
Fun with Magnetic Fields
Essential to many high-tech devices, magnetism can help you create a pseudo perpetual motion machine. Glue a magnet to each side of a wooden box whose sides are no more than three inches. Ensure that the same pole of each magnet faces inward towards the box's center. After you glue another magnet to the box's bottom, place a transparent lid on the box. Cover a final magnet's pole with clay to block its magnetism. The exposed pole-- the one opposite of the pole you covered-- must be the one the box's magnets repel. When you drop the magnet into the box, the magnet will jump around, repelled by all the other magnets. When it jumps upward, gravity will bring it down, bringing it close to the attached magnets, which will repel it back upward, and so on.