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. A seesaw is a lever, and a ramp is an inclined plane. 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 compound machine is made of two or more simple machines.

A lever is a bar that turns around a pivot point called the fulcrum. Scissors are an example of a lever. So are 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. Bend your arm and watch the elbow turn. Then think about a seesaw, which is a lever too.

Stairs are an inclined plane, and so is a ramp. Pushing an object up a ramp takes less force than lifting it straight up. But the path is longer. That is the trade. 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 too. It is wrapped around a cylinder.

A wedge pushes things apart or cuts them. It is made of two inclined planes put back to back. A doorstop is a wedge. 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. It turns a twisting motion into movement forward or backward. Ask a grown-up to help with sharp or heavy tools.

Now look at two machines that use wheels. 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. Using more than one pulley lets you lift a heavy load with less force. A wheel and axle helps move a load by turning or rolling.

Mechanical advantage tells how many times a machine multiplies the force you put in. A longer lever handle, a ramp and more than one pulley all let you use less force. Yet a simple machine cannot create energy. It can let you use less force, but you must move over a longer distance. That rule holds for every simple machine here.

Words to know

• fulcrum: the pivot point
• wedge: pushes things apart
• pulley: wheel with rope
• compound: made of parts
• advantage: force multiplied