The word machine comes from a Greek word meaning to help make things easier. That fits the six simple machines: the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw. Simple machines make work easier, but they do not decrease the amount of work you have to do. A simple machine has no internal source of energy, and a simple machine cannot create energy. A seesaw is a lever, and a ramp is an inclined plane.

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. Pushing against a wall that does not move is not work in the physics sense, even though it is tiring. 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 simple machine can let you use less force, but you must move over a longer distance.

A lever is a bar that turns around a pivot point called the fulcrum. A seesaw, a pair of scissors and a wheelbarrow are all levers, and so is your forearm, with the elbow as its pivot. A longer handle on a lever lets you do a job with less force. A wheel and axle is a kind of lever, and it helps move a load by turning or rolling. A pulley is a wheel that a rope goes over.

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 a ramp makes it easier to move something upward, but the path is longer. Ramps were probably used when the ancient Egyptians built the pyramids, to move large blocks of stone to the top. A screw is an inclined plane wrapped around a cylinder, and a wedge pushes things apart or cuts them. Ask a grown-up to help with sharp or heavy tools.

A wedge is made of two inclined planes put back to back, so both a wedge and a screw put an inclined plane to work. A doorstop is a wedge. A screw helps to hold things together, and it turns a twisting motion into movement forward or backward. A jackscrew can raise very heavy things, such as a house.

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. An old barn's roof overhang is where a pulley was once attached, to lift hay bales into the loft.

A compound machine is made of two or more simple machines. Some compound machines are clippers, a manual pencil sharpener, a crane and a bulldozer. Bicycles include wheel and axles, levers, screws, and pulleys. Mechanical advantage tells how many times a machine multiplies the force you put in. Friction is a force that resists the motion of two objects sliding past one another, and in real machines, friction turns some of the work put in into wasted heat.

Words to know

• fulcrum: the pivot point
• friction: resists sliding motion
• compound: made of parts
• advantage: force multiplied
• force: push or pull