Skimmer Regatta 3

Air Resistance and Motion

Students decide which shapes reduce the most air friction

• Define drag as the force that slows an object's movement through fluid (air or water0.
• Define a streamlined shape as one that reduces drag by letting fluid flow smoothly around it.
• Experiment with making the skimmer more streamlined and use data to choose the shape and size of the skimmer's front and back so that it moves as far as possible with the given air supply.

When an object moves through air, the air pushes against it, slowing the object's movement. The force that slows an object's forward motion is called drag. Drag is a form of friction. One way to reduce drag is to design an object with a streamlined shape that allows air to flow around it easily. Streamlined shapes re not pushed back easily by air so they cause less drag. Conversely, shapes that block the smooth flow of air cause the air to swirl or eddy, around them, causing drag.

In Explore, students attach paper (cut in various shapes) to the front of a toy car and measure the effects of drag on each. Generally, the larger the shape's surface, the greater the drag and the shorter the distance the car will travel. In Investigate, students test body designs for their skimmers to find which shapes make the skimmers more streamlined. While some shapes will appear to make the skimmer more streamlined, shapes that are too large may add too much weight to the skimmer and reduce the distance it can travel.

Have students complete Steps 1-5 of the Explore activity in their teams. Be sure every team tries at least four different shapes on the front of the car.

Have a student read Challenge! aloud. Tell students they have five minutes to work on the Challenge! activity. After five minutes call an all-class Think Tank meeting.

1. Have one student from each team draw on the chalkboard the shape that slowed the car the most and the shape that slowed the car the least. Explain that as the car moves down the ramp, air pushes on it. If an object moving through air has a large surface that blocks the flow of air (such as the square attached to the toy car in Step 3), the air will slow the object. Discuss why some shapes slowed the car more than others. (See Science Background Capsule.)
2. Explain that the force that slows an object moving through the air is called drag. Write on the chalkboard, "Drag is the force that slows an object's movement through air." Invite students to create a poster illustrating this statement.
3. Ask students to show how they attached the paper in the Challenge! So the car did not slow down as much and traveled farther. Help them see that having just an edge of the paper moving into the wind (for example, on the side or top of the car) provides a smaller surface area for drag to act upon. This lets the car go faster and travel farther.
4. Ask students to name some familiar vehicles that produce lots of drag or a little drag. For example, ask them to compare the shapes of sports cars and mail trucks. [Lots of drag: square fronts such as trains, subway cars, old vans; a little drag: boats, submarines, airplanes, race cars, rockets, vans with angled fronts]
5. Tell students that drag can be reduced by making an object streamlined. An object with a streamlined shape can move more easily because air flows around it more easily. Engineers design airplane wings with a rounded front edge that air can move over easily. Ask students to describe the shape of boats and ships they have seen. Suggest that their skimmers might move farther if they are streamlined like most ships and boats.
6. Have one student read the introduction of the investigate section. Check for students' understanding. Tell students to return to their teams and investigate how to make the simmers more streamlined. Remind students to read each numbered question, experiment to find the answer, and copy and complete the Design Choice Record. You may choose to have students use the record sheets found in the Skimmer Regatta Design Team Log.

Circulate among teams to check on their progress. Encourage cooperative problem solving, creative experimentation, and neat and thorough record keeping. Make sure everyone gets to participate.