Simple machines, such as levers, pulleys and screws, demonstrate principles of physics that are part of a grade school science curriculum. With supervision, these machines are safe to build and operate.
Some of the most basic machines are the lever, inclined plane and pulley. Kids can easily build these machines themselves, or find household versions of them to demonstrate physical principles. For example, a lever can come in the form of a hammer pulling a nail out of the wall. The hammer acts as the fulcrum, strengthening the force coming from a person's arms enough to pull the nail.
An inclined plane teaches children about the physical principle of work. Work is equal to force multiplied by distance. When trying to lift a heavy object, adding distance by placing it on an inclined plane decreases the amount of force that is required compared to lifting it straight up. Slides and wheelchair ramps are examples of inclined planes.
A pulley is a man-made machine that changes the direction of a force. If a heavy object is tied to one end of a pulley it can be lifted up by someone pulling down on the other end of the rope. Pulleys are often found on clotheslines and construction cranes.