1. Find out students' ideas about gravity. Ask the following:
- What is gravity?
- Where is gravity?
- What does gravity do?
Give students time to explain their ideas. Record their thoughts on the board or on a piece of chart paper, so that you can return to them later.
2. Hold up a hammer and a feather and ask students to predict what would happen if you dropped them simultaneously from the same height: Would they hit the ground at the same time or at different times? Do not drop the objects at this point. Show students the Galileo on the Moon video. After screening it, ask the following:
- Did you expect the hammer and the feather to land on the surface of the Moon at the same time?
- Why do you think this happened?
3. Try investigating some of these questions about gravity. Ask students to predict what would happen if you dropped a whole apple and half an apple at the same time from the same height: Would they hit the ground at the same time, or would one hit before the other? Why? Have the students record their predictions and explain their thinking. Ask students to share some of their predictions. Then drop the apples. Allow time to discuss the results and for the students to try to explain the factors that produced them. Use this activity as an opportunity to discuss gravity as a force that pulls objects toward Earth.
4. Go to the Galileo: His Experiments interactive activity (Falling Objects experiment). Ask students to predict which cannonball will hit the ground first and give reasons for their prediction. Select their choice to see if their prediction was supported or not supported. Hopefully, at this point, students are willing to accept or at least consider the idea that all objects fall at the same rate, regardless of their mass. Galileo conducted several experiments and concluded that the effect of gravity on earthly objects is the same, regardless of the mass of those objects. He argued that in the absence of other forces such as air resistance, all falling objects accelerate toward Earth at the same rate.
5. Show the Galileo on the Moon video again. Remind students of the predictions they made in step 2 (would the hammer and the feather hit Earth at the same time). Try it. Then ask:
- Why did the hammer and the feather fall at the same rate on the Moon but not on Earth?
Introduce the idea of air resistance, a force (friction) that opposes any object moving through air. Ask:
- What role did air resistance play in the rate at which the objects fell?
6. Show the video What Is "Weightlessness"?. This demonstration can be interpreted as the water floating inside the cup, but from Galileo's experiments, we know that the water and cup are falling at the same rate even though their masses are different. Review what happened in the segment, and ask:
- Were you surprised that the water stopped pouring out of the holes in the cup once the cup started to fall?
- Can you think of an explanation for this based on your understanding of the way falling objects are affected by gravity?
- Why do you think the term weightlessness is used in the title of the video? (optional)
7. Have students try the falling cup activity from step 6 in your classroom. Experiment with a variety of liquids. Ask students to first predict the results. Do they think they will get the same result no matter which liquid is used, or a different result? Ask them to explain their reasoning; see how well they apply what they have learned from previous investigations to these new situations.