• During the lesson, monitor students’ understanding as they build upon each step in the lesson. Use the following checklist to assess students’ understanding of forces:

• Student understands that the balloon’s movement is the result of the net forces acting on it.

• Student understands that net force is the summation of all forces.

• Student understands why the balloon is unlikely to move in a straight line.

• Student understands that force and the resulting movement can be broken into separate vectors for analysis.

1. Show the class some of Galileo’s sketches and images of Jupiter’s moons found at http://www.aip.org/history/cosmology/ideas/images-ideas/discovery-galilean-new-sml.jpg. Lead students in a discussion of how this planet and its moons were discovered. “Who discovered Jupiter in the night sky?” “How did he know it wasn’t a star?”

Next, show students a picture of Jupiter and its moons, taken from NASA’s Galileo mission at http://solarsystem.nasa.gov/galileo/gallery/index.cfm. “How did we get close enough to take these pictures?” Collect ideas from the students and lead students toward asking about the specifics of space travel. “It’s just a matter of pointing the space craft in the right direction, isn’t it?”

Postpone students’ curiosity for the time being as you start your lesson. Remember to address this last question after the narrative.

“When NASA sends a probe to another planet, there is no single calculation that can tell the mission planners where the spacecraft is going to be at any one time. Especially when the spacecraft gets close to another body, such as a moon or planet, mission planners must calculate the forces acting on it from one second to the next. The net of all those forces, plus its existing momentum, determine where the spacecraft will go from moment to moment. Tiny changes at one point can (hours, days, or even months later) lead to a completely different orbital path, and make the difference between attaining orbit, crashing on the surface, or being lost in space.

“Here on the surface of the Earth, the situation is usually less extreme. But we are still being constantly acted on by multiple forces. The situation is actually more complex than the one faced by NASA mission planners, since the forces at work in space are known in advance and their affect can be calculated with precision. In daily life the forces at work are not always predictable. Even in those cases, however, the net force is usually easy to measure, because of the resulting movement.

“The forces acting on objects here on the surface of the Earth usually boil down to gravity and friction, plus momentum. However complex they are, when they are in balance, the body in question is stationary. When they are not in balance, the body moves in the direction of net force.”

Return to the earlier discussion and again ask students, “It’s just a matter of pointing the spacecraft in the right direction, isn’t it?”

2. Display a toy helium balloon tethered on a string.

“The vertical forces on the balloon are balanced. The force of gravity is more than offset by the buoyancy of the helium it contains. The additional buoyancy is balanced against the force imposed by the string.”

Add paper clips to the string until the balloon no longer needs to be tethered.

“The balloon’s buoyancy is now balanced by gravity. It is in a condition called neutral buoyancy. Any vertical motion will have to be the result of air currents, until the balloon either leaks a little helium to become less buoyant or warms enough to become more buoyant.

“Horizontally, any motion will result in air currents pushing against the balloon harder on one side than on any other side. Additionally, the currents have to be pushing hard enough to overcome the balloon’s own air resistance. In a complex room like this, predicting what the effects of the air currents will be is probably impossible, but we can gauge the net result.”

3. Ask students to predict the place the balloon will go after 60 seconds, if it is released with the fan on, with an explanation of their prediction.

4. Place a marker where the balloon’s string is hanging, have a volunteer turn on the fan, and have another volunteer start a one-minute timer.

The balloon will drift about the room in response to changing air currents as the air from the fan bounces off walls and objects.

After 60 seconds, mark the spot where the balloon’s string is now hanging.

5. Lead the class in a discussion of why the balloon moved as it did, and what forces must have been acting on it. The forces acting on it were erratic, but the net force acting on it caused it to move from the start point to the end point.

Review Newton’s first law of motion: that a body in rest or motion will remain at rest or in motion until acted on by a net force. The balloon did not appear to follow the law, starting and stopping and moving for no obvious reason. There are two reasons for this:

• As it moves, the balloon encounters so much air resistance (or drag) that constant force is required to keep it moving.

• The balloon’s mass is so small that its momentum, in the face of the air resistance, is not a factor.

When a fan blows air against the balloon it moves with the direction of the wind, the force of the wind is not balanced by any other force, causing the balloon to move. The net force is in the direction of movement.

When the fan stops blowing, the air resistance that the moving balloon encounters is no longer balanced by the force of the wind. Its own momentum is soon overcome by air resistance, and the balloon stops moving. All forces are balanced and there is no net force.

6. Lead the group in using graph paper (S-7-1_Graph Paper.doc) to plot the net force that acted on the balloon during the 60 seconds that it was free. Students should make a schematic of the room, showing approximately where the balloon started and finished. They will plot an X and Y axis showing its two vectors of movement, and a diagonal line between the start and end points.

The balloon probably did not travel in a straight line between the two points, but that fact is disregarded, as only the net force is of interest.

Students will then calculate the balloon’s velocity between the start and end points, and its velocity along the separate X and Y axis. They will add that to the chart.

The results may resemble this example, with the distances posted in meters and the velocity figures in centimeters per second:

7. Place the fan in a different spot, or face it in another direction, and repeat the activity. Release the balloon and observe for 60 seconds, marking its start and stop locations, and measuring the separation. Students will then individually graph the results, figuring the two vectors of movement.

Extension:

• Students who may be or are going beyond the standards can run the experiment several times while rearranging the location of the fan. Each time they should predict the end location of the balloon, and then plot its actual vectors.

• The experiment can also be performed as a game, with students making predictions of the balloon’s locations after 60 seconds.