There are 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 a simple machine cannot create energy. What it can do is let you use less force, as long as you move over a longer distance. A seesaw is a lever, and a ramp is an inclined plane. A compound machine is made of two or more simple machines.

A lever is a bar that turns around a pivot point called the fulcrum. A seesaw is a lever, and so are scissors, a wheelbarrow and a crowbar. Your forearm is a lever too, 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 bicycle is an example of a wheel and axle.

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 also an inclined plane, wrapped around a cylinder, and it shows the same trade of less force for a longer distance.

A wedge pushes things apart or cuts them, and it is made of two inclined planes put back to back. A doorstop is a wedge, and so is a knife. A wedge can also be used to split a log. A screw does a different job: it 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. Ask a grown-up to help with sharp or heavy tools.

A pulley is a wheel that a rope goes over. 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.

Mechanical advantage tells how many times a machine multiplies the force you put in. There is also a catch. In real machines, friction turns some of the work put in into wasted heat. Simple machines make work easier, but they cannot create energy. A machine can let you use less force, but you must move over a longer distance.

Words to know

• fulcrum: the pivot point
• friction: work into heat
• compound: made of parts
• advantage: force multiplied
• jackscrew: screw that lifts