Skimmer Regatta 5

Mass and Balance

Students decide whether adding pennies will stabilize the skimmer or merely slow it down.

• Define mass as the amount of material an object is made up of.
• Experiment and use data to decide if adding stability to the skimmer by adding mass will help it travel as far as possible with the given air supply.

Mass is the amount of material an object is made up of. Weight is a measure of the pull of the force of gravity on the mass. On Earth, weight and mass are essentially the same. Every object has a point on which it can be held in balance. This balancing point is called the object's center of gravity. In general, the lower and object's center of gravity, the more stable the object is. Therefore, adding mass to the base of an object around the balancing point make the object more stable. However, adding mass unevenly to an object can change the object's center of gravity and make the object less stable. If a significant mass is free to move around on a moving object, the object's center of gravity keeps changing and can make the object unstable and interfere with forward motion. In Explore, students add mass in the form of marbles to a milk carton to make it more stable. Students also find the balancing point of a ruler with equal and unequal mass at the ends.

In Investigate, students apply what they've learned about balance and adding mass for stability to the skimmer. Students will experiment to decide if adding pennies affects the skimmer's stability and the distance it travels. Two pennies may add stability and make the skimmer move straight but decrease the distance the skimmer travels. Results will vary from skimmer to skimmer because of various weight and size of sails, etc.

Have students complete Steps 1-5 of the Explore activity in their teams. Be sure students find the ruler's balancing point before they add pennies.

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

1. Define mass. Write on the chalkboard, "Mass is the amount of material an object is made up of." Explain that adding mass (marbles) to the carton in Explore made the carton more stable and, as a result, more difficult to blow over. Adding the paper towel kept the mass from moving. This fixed mass made the carton even more difficult to move.
2. Have a student from each team report the measurement at which they balanced the ruler in Step 4 of Explore. Tell students that this measurement is the ruler's balancing point. Ask student show they kept the ruler balance when they added the penny. [Added a penny to the other side.] Tell students that for an object to stay balanced, the mass added to each side of its balancing point must be equal.
3. Ask for the measurement at which students balanced the ruler in the Challenge! activity. Explain that because the mass added to both sides was not equal, the balancing point changed.

Ask a few students to describe how a teeter-totter works. Guide them to say that it works best when the two people are the same weight. Discuss what happens when people of different weight play on a teeter-totter. [Heavier person stays on the ground.] To make the teeter-totter work, the heavier person must move closer to its center. Therefore, changing the distance between the balancing point and the heavier child (heavier load) can also balance a teeter-totter.

4. Have students try to find the balancing point of the skimmer by balancing it (upside down) on the eraser end of a pencil. Tell students that engineers must think about an object's balancing point when they design machines. For example, a car's center of gravity is near the bottom and centered between the wheels so the car doesn't tip over. Tell students that when engineers design a machine that will carry a load, they must choose where to put the load to keep the machine moving as smoothly as possible. The must decide how heavy a load the machine can carry and still travel far.
5. Have students imagine a rowboat with many people aboard. Ask them if they think the people should all sit at one end or one side. Have students explain their answers and tell where people should sit.
6. Have one student read the introduction of the Investigate section. Check for understanding. Tell students to return to their teams and investigate to find the best way to make the skimmer travel as far as possible. Point out that adding weight for stability may or may not help the skimmer go far. Remind students to do three trials to test each choice, record their data and observations, and copy and complete the Design Choice Records. You may choose to have students use the record sheets found in the Skimmer Regatta Design Team Log.

Circulate to check on teams' progress. Since this is the last card before the Regatta, you may want to provide each team with a Design Choice Record summary form that has them state all their final choices.