The word machine comes from a Greek word meaning to help make things easier. There are six simple machines: the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw. Most of them were in common use for centuries before Leonardo da Vinci's time, but he experimented with them and improved them. Simple machines make work easier, but they do not decrease the amount of work you have to do, and a simple machine cannot create energy. A seesaw is a lever and a ramp is an inclined plane, although some engineers call gears the seventh simple machine.

A force is any push or pull. In science, each time a person pushes or pulls an object and moves it, that person does work, whereas pushing against a wall that does not move is not work in the physics sense, even though it is tiring. Work is force times distance. Force is measured in newtons, distance in meters, and work in joules, so a force of 8 newtons that lifts a ball 2 meters does 16 joules of work. Although a simple machine cannot change the amount of work, it can change the force you apply and the distance over which you apply it. No machine can output more energy than was put into it.

A lever is a bar that turns around a pivot point called the fulcrum, and Archimedes developed the law of the lever with his Treatise on the Balance. There are three classes of levers, set by where the effort, the load and the fulcrum are placed. In a first-class lever the fulcrum is between the load and the effort, and seesaws and crowbars are examples. In a second-class lever the load is between the fulcrum and the effort, and a wheelbarrow is an example. The ideal mechanical advantage of a lever is the length of the effort arm divided by the length of the resistance arm, so a longer handle on a lever lets you do a job with less force.

A ramp is an inclined plane, and so are stairs. Pushing an object up a ramp takes less force than lifting it straight up, and in one worked example, using a ramp cut in half the force needed to raise a ball. The more gradual a ramp's slope, the greater its mechanical advantage, but the load must move farther, and rollers used with an inclined plane can raise its efficiency by reducing friction. Ramps were probably used when the ancient Egyptians built the pyramids, to move large blocks of stone to the top. A simple machine can let you use less force, but you must move over a longer distance.

An inclined plane and a wedge are two forms of the same simple machine, and a wedge is made of two inclined planes put back to back. A wedge pushes things apart or cuts them, and it turns a push in one direction into a splitting motion at right angles to the blade, and a wedge can be used to split a log. A screw is an inclined plane wrapped around a cylinder, and it turns a twisting motion into movement forward or backward, while a screw that is turned once advances by the distance between adjacent screw threads. A screw helps to hold things together. Ask a grown-up to help with sharp or heavy tools.

A pulley is a wheel that a rope goes over, and a flagpole uses a pulley to raise the flag. A single fixed pulley has an ideal mechanical advantage of 1, but lets you redirect the force so you can stand clear of the load. Using more than one pulley lets you lift a heavy load with less force. For a pulley system, count the ropes holding up the load, and that number is the ideal mechanical advantage. To raise a load 1 meter with a pulley system, you must pull N meters of rope, where N is the number of supporting ropes. A wheel and axle helps move a load by turning or rolling, and any crank-operated device is an example of a wheel and axle.

Friction is a force that resists the motion of two objects sliding past one another. It always acts in the direction opposite to movement, and its strength depends on the type of surfaces in contact and how hard they are pressed together. In real machines, friction turns some of the work put in into wasted heat. Mechanical advantage tells how many times a machine multiplies the force you put in. The ideal mechanical advantage is that of a perfect machine with no loss of useful work to friction between moving parts. In the ideal case, the work put into a simple machine equals the work it puts out.

Words to know

• fulcrum: the pivot point
• friction: resists sliding motion
• joules: units of work
• advantage: force multiplied
• ideal: perfect machine's