Simple machines are mechanical constructs that increase a simple force when applied to a load, or change the direction of that force. All compound machines are made of combinations of simple machines. Traditionally, the six basic simple machines are the inclined plane, lever, pulley, screw, wedge, and the wheel and axle. Third grade students are able to understand these machines and how they work by performing science experiments involving them.
The Inclined Plane
To help third graders learn about the inclined plane, have them move weights with and without the help of an inclined plane. Attach a moderately heavy weight to a spring scale, and have the children lift the weight directly from the floor to a low table. The spring scale measures the amount of force needed to move the weight straight up with no help. Next, attach a ramp made of heavy cardboard or wood from the table to the floor. Use the spring scale to pull the weight up the ramp to the table and measure the lesser amount of force needed.
Levers are simple machines placed on a fulcrum that help magnify force to move the load positioned at the end of the lever. Third-graders can enjoy understanding the lever by building a simple catapult that shoots coins or other light-weight objects into the air. To do this, balance a ruler on a pencil, and place a coin on the end of the ruler to serve as the load. Drop a light weight or another coin on the high end of the ruler, and watch as the load jumps. Try the same experiment with different sized weights, dropping weights from different heights, and with the fulcrum in different positions.
Many school children see a pulley at work every day when their school's flag is raised and lowered. You can help third-graders realize the part the pulley plays by taking them to the flagpole. Ask them to jump to the top of the flagpole to attach the flag. Of course, they won't be able to do it, but they'll have fun trying. Have them attach the flag and pull it up, noting that as they pull down, the flag goes up, showing how the pulley changes the direction of the force.
Third-graders can understand that a screw is simply an inclined plane wrapped around a center pole by creating their own screw shape. Have them cut long thin triangles of paper. If they've already done a project about the inclined plane, they should be able to explain how the paper could be an inclined plane. Have them wrap the paper around a pencil, then pull the pencil out. The paper will form the shape of a screw.
By third grade, children are often already familiar with the wedge as a simple machine because of the building blocks they play with. Have the children stand two rectangular blocks on edge side by side, touching each other. Then have them push a wedge-shaped block between the two blocks and see what happens as they split apart. Try the experiment with different widths of wedges to see the different reactions.
The Wheel and Axle
Children see wheels and axles at work every day, whether in the form of their bicycles or as the doorknob they turn to leave their bedrooms. To help them see how a wheel and axle work in a simple way, give them each a screw, a screwdriver and a piece of soft wood. Have them try to press the screw into the wood with their hands; they won't get very far, and it will take a lot of effort. Then have them use the screwdriver, explaining that the handle of the screwdriver is a wheel. When they turn the wheel, the axle, which is the screwdriver itself, applies increased force to the screw, making it easy to push into the wood.