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 has no internal source of energy. No machine can output more energy than was put into it. A seesaw is a lever, and a ramp is an inclined plane.

A force is any push or pull, and in science, each time a person pushes or pulls an object and moves it, that person does work. 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, and 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.

A lever is a bar that turns around a pivot point called the fulcrum. A seesaw is a lever, and a longer handle on a lever lets you do a job with less force. A wheel and axle is a kind of lever that helps move a load by turning or rolling. A pulley is a wheel that a rope goes over, and a flagpole uses a pulley to raise the flag. A single pulley changes the direction of a force, but 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.

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. Ramps were probably used when the ancient Egyptians built the pyramids, to move large blocks of stone to the top. An inclined plane and a wedge are two forms of the same simple machine, and a simple machine can let you use less force, but you must move over a longer distance. Ask a grown-up to help with sharp or heavy tools.

A wedge is made of two inclined planes put back to back, and it pushes things apart or cuts them. A wedge turns a push in one direction into a splitting motion at right angles to the blade. A doorstop is a wedge, a knife is a wedge, 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 also helps to hold things together. A compound machine is made of two or more simple machines.

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 is the ratio of the output force to the input force of a simple machine, and it tells how many times a machine multiplies the force you put in. A simple machine cannot create energy. It can let you use less force, but you must move over a longer distance.

Words to know

• fulcrum: the pivot point
• friction: resists sliding motion
• newtons: units of force
• joules: units of work
• advantage: force multiplied