This is the fifth and final regular Investigation of the LabLearner CELL Our Solar System. In it, students will explore the forces responsible for planetary motion and orbits.

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A. Invite students into a conversation about the movement of the planets.

1. Ask students: What are the two ways you have learned that planets move? Planets rotate and revolve.

2. Ask students: How does the Earth move around the Sun? The Earth rotates on its axis as it revolves around the Sun.

3. Ask students to imitate the movement of the Earth around the Sun by revolving and rotating around their desks.

4. Ask students: What is the shape of a planet’s orbit? Orbits are elliptical. The Earth’s orbit, for example, is a slight ellipse so that it is nearly that of a circle.

5. Ask students the following questions to begin thinking about factors that cause orbits:

Ask students: What makes the planets move around the Sun?  Student answers will vary. Encourage varied answers.

Ask students: What keeps the planets in their orbits? Student answers will vary.

6. Tell students that they will investigate these questions in the lab. To prepare to answer these questions, guide students in the following activities focused on two new ideas: forward motion and gravity.

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B. Begin discussing forward motion.

1. Ask students: Do all planets move? Yes. Every planet revolves and rotates.

2. Tell students that every planet is moving forward. In other words, every planet has forward motion, because the orbit and direction of travel in the orbit do not change from one revolution to the next.

3. Divide the class into groups of three. Instruct each group to create one demonstration of something moving forward, or an object having forward motion. Allow several minutes for brainstorming and offer classroom resources and materials for their demonstrations.

4. Encourage students to demonstrate forward motion to the class. Student demonstrations may include: pushing a pencil, moving their hands through the air, walking forward, and so forth.

5. Instruct all students to stand up beside their tables or desks. Ask them to walk forward until they reach the front of the room. Invite students to imagine that the classroom didn’t end and that they could continue walking forever. 

Ask students: What type of motion did you have in order to walk to the front of the classroom? Students had forward motion.

6. Explain again that every planet has forward motion. 

Ask students: If every planet has forward motion, why don’t they continue to move in a straight line forever? Student answers may vary. Encourage varied responses.

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C. Introduce gravity to the class. The following activities may be helpful:

1. Pick up a book and hold it in the air at the level of a nearby desk or table. 

Ask students: What would you predict will happen if I let go of this book? Students will likely respond that the book would fall.

2. Drop the book. Point out that students’ predictions were correct.

3. Ask students: Why did the book fall? Student answers may vary. Some students may suggest that gravity made the book fall.

4. Tell students that gravity is something called a force, and a force is a push or a pull. Gravity is a pull that exists between two objects. Gravity causes objects to be pulled toward other objects. Very large objects, like the Earth, have a lot of gravity. This gravity causes things to be pulled toward the center of the Earth. The Earth exerts gravity on people.

5. Explain that the bigger an object is, the more force it applies to other objects to pull them towards it.

a. Ask students, Which is bigger – the Sun or the Earth? Students should recall that the Sun is bigger than the Earth.

b. Ask students, Which is bigger – the Moon or the Sun? Students should recall that the Sun is bigger than the Moon.

c. Ask students, Which would result in a greater force of gravity, the pull of the Sun on the Earth or the pull of the Moon on the Earth? Student answers may vary. Guide students to realize that the Sun exerts a larger force of gravity on the Earth than the Moon or the other planets in the Solar System.

6. Ask students to imagine that the Earth did not pull objects toward it. 

Ask students: How would this classroom be different if gravity did not exist? Encourage students to think creatively. Students should realize that tables and chairs would not rest on the floor. Pencils and books would not rest on desks.

7. Ask students: If large objects have a strong force of gravity, do you think objects other than the Earth have a strong force of gravity? Encourage students to think of other parts of the Solar System. Student answers may vary. Students should think of larger objects such as other planets and the Sun.

8. Tell students that just as the Earth acts to pull things toward it because of the force of gravity between the Earth and other planets, the Sun pulls things toward it because of the force of gravity between the Sun and other planets. 

Emphasize once again that because the size of the Sun is so large, the force of gravity between the Sun and other planets is much larger than the force of gravity between the Earth and the Moon, for example.

9. Tell students that in the lab they will investigate how the Sun’s gravity affects the orbit of the planets.

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D. Perform a final demonstration to prepare students for the investigation in the Lab.

Note: If students’ desks are large, this model may flow more smoothly using a chair instead of a desk.

1. Instruct students to stand beside their desks and again model forward motion by walking three or four steps past their desks.

2. Instruct students to next imagine that they are the Earth and their desks are the Sun. 

Ask students: What effect does the Sun have on the Earth’s movement? The Sun’s gravity pulls the Earth toward it.

3. Tell students to illustrate the Sun’s gravity by placing a hand on their desks. Because the Sun’s gravity always pulls the Earth toward it, it does not allow the Earth to move away from it. To model this, students must keep a hand on their desks at all times.

4. Instruct students to again move forward, this time remembering to keep a hand on the desk. As students begin walking, encourage them to brainstorm how to keep a forward motion while keeping a hand on the desk.

5. If no students have begun to move in a circle around their desks, encourage students to do so. 

Ask students: Are you moving forward? Student answers may vary. 

Point out to students that although they are not moving in a straight line, they are moving forward.

6. Ask students: Are you able to keep your hand on the desk while moving around it? Yes, students are able to keep a hand on the desk as they move around it.

7. Tell students that the gravity of the Sun and the forward motion of the planets combine to create something similar to the model. Explain that students will perform and investigate other models of the movement of the planets in the lab.