Resource: Trash Bag Kites

Children and adults around the world have been enjoying kites for thousands of years. In some ways, kites are like flying sailboats; both are designed with sails that respond to changes in the wind. In other ways, they are like gliders, held aloft by nothing more than moving air. A combination of pushes and pulls keeps kites in flight. Kites that do not adjust to these forces will fall to the ground.

The natural tendency of any airborne object over Earth is to fall to the ground. The force that causes this to happen, called gravity, acts on an object by pulling it toward Earth's center. The force that stops a flying object from falling is called lift. Even though we cannot see air, we know it is present because we can feel it when the wind blows. The moving air we sense on our skin or that causes a flag to flap contains energy. Lift is the force generated by wind energy that pushes up against the bottom of the surface area of bird or airplane wings, or a kite's covering material.

Just as having too little lift allows gravity to bring a flying object crashing to the ground, having too much lift can cause the opposite to occur. What do you think would happen if you let go of the kite's string? It would fly away with the wind.

Well-designed kites manage gravity and lift, as well as the other forces that act on them. Like an airplane, kites have a solid frame. This frame may be made from plastic or wood, and is covered by a sail. In a common diamond kite design, like those made by the children featured in this video, the sail spans the spine (which runs the length of the kite) and the cross spar (which runs from wingtip to wingtip). To perform well, a kite needs to be light enough to be lifted off the ground and its parts strong enough to withstand high winds. Although some metals are stronger than most wood and paper, they are typically too heavy to be used in kites. Depending on how the parts are modified and assembled, kites may sacrifice some stability for maneuverability.

To see how moving air helps other vehicles to overcome forces and move, check out Designing a Puff Mobile.

• In general, what are the properties of kite-making materials?
• Does a kite have to have a certain shape in order to fly? What kinds of shapes will allow a kite to fly?
• Why does a kite usually have a tail? What does the tail do?
• What force is pushing on the kite? What force is pulling on the kite?
• Explain the pushes and pulls on the kite when the wind dies down. What do you predict will happen to the kite?