Compound machines are comprised of two or more simple machines. Simple machines, which include levers, inclined planes, screws, wedges, pulleys and a wheel-and-axle, are mechanical devices that use the simplest possible mechanisms to transform force. Although compound machines are usually covered in physics, they can be simplified enough that even elementary students can see how they work.
Setting up a double pulley is an easy way to teach elementary students about the benefits of compound machines. Set up one pulley in the classroom and have students practice lifting a weight using it. Then, thread the rope of the first pulley through a second, and have students practice lifting the same weight. By adding a pulley, less force is required to lift the weight. Give each student a chance to practice using the compound machine to lift the weight, and talk about how a double pulley can make a job easier.
Toy Car Track
This is a simple project that many elementary students have already done if they enjoy racing toy cars. Show students how toy cars have a wheel and axle that allow them to roll forward, and have students play with rolling cars along a flat surface. Then, set up a simple inclined plane by elevating one end of a board with a few blocks to make a slope. Set the toy cars at the top of the plane, and see how much more quickly they roll down compared to the flat surface.
Have elementary students play with the classroom dust pan and broom to demonstrate how a lever and inclined plane work together to scoop up toys. Show students how the broom handle works as a lever by sweeping up small toys, and have them play with sweeping the floor of toys. Then, have students look at the dustpan and notice how it's really just a small inclined plane, like the race car track they built earlier. Combine the lever and inclined plane of the broom and dust pan to easily sweep up toys.
A wheelbarrow is a great example of a compound machine that many elementary schools already have. Get a toy wheelbarrow in the classroom and show students how the wheel works to move the wheelbarrow forward, while lifting the wheelbarrow is like using a lever. Have students play with moving loads in the wheelbarrow and compare the amount of work it takes to move a stack of books from one of the classrooms to the other by carrying them vs. using the wheelbarrow.