## The Pull of the Planets

### Overview

The Pull of the Planets is a 30-minute activity in which teams of children model the gravitational fields of planets on a flexible surface. Children place and move balls of different sizes and densities on a plastic sheet to develop a mental picture of how the mass of an object influences how much effect it has on the surrounding space.

This activity should be conducted after Heavyweight Champion: Jupiter!, which allows the children to discover the force of gravity in the solar system. These concepts involve more advanced science than previous activities in Jupiter's Family Secrets, and they explore more deeply the science of the Juno mission and the rich information it will return to us. Facilitators who choose to undertake this activity should have a firm grasp of the scientific basis so that misconceptions are not introduced to the children.

This series is appropriate for children ages 10 to 13.

### What's the Point?

• Gravity is the force that keeps planets in orbit around the Sun. Gravity alone holds us to Earth's surface.
• Planets have measurable properties, such as size, mass, density, and composition. A planet's size and mass determines its gravitational pull.
• A planet's mass and size determines how strong its gravitational pull is.
• Models can help us experiment with the motions of objects in space, which are determined by the gravitational pull between them.

### Materials

For each group of up to 30:

For each group of four children:

• 1 (20" by 12" or larger) embroidery hoop
• Something to support the edges of the embroidery hoop, such as foam bricks or books
• 1 thin stretchable plastic sheet, like a plastic garbage bag or sheets of plastic-wrap
• 2–4 (1/2"–wide) small marbles
• 1 (2") Styrofoam™ ball
• Half a can of Play-Doh©

For each child:

For the facilitator:

### Preparation

• Review the complete background information and the Facilitator's Guide to Gravity.
• Prepare the gravity fields: stretch the plastic sheets (plastic wrap or garbage bags) around the inside of the embroidery hoops, then add the outer hoop, keeping the plastic stretched tightly.
• Set out the remainder of the materials.

### Activity

1. Ask the children to connect what they have learned about gravity to the motions of objects in the solar system.

• Ask the children to recall from Heavyweight Champion: Jupiter! Which properties cause a planet to have more or less gravity? Planets that are massive and have the largest diameters have the most gravity. Which properties do not influence gravity? The presence of an atmosphere, temperature, and distance from the Sun do not affect a planet's gravity.
• Are the objects in the solar system still or are they in motion? The Sun's gravity pulls the planets in orbit around it, and some planets pull moons in orbit around them. Even spacecraft are in motion through the solar system, either in orbit around the Earth or Moon, or traveling to further worlds, because of gravitational forces. The Juno mission will be pulled into orbit around Jupiter by Jupiter's intense gravity.
• How does gravity influence the movements of objects — such as planets — in the solar system? Has anyone seen or played with a "gravity well?" How does a "gravity well" model gravity in the solar system — what part of this model is the Sun? The planets? The center of the gravity well is the Sun, and the coins or marbles are a model of the planets. The closer the planet is to the Sun, the greater the pull of the Sun's gravity, and the faster the planet orbits. This model fails in that objects in stable orbits do not fall into the Sun. (Comets are objects with orbits that can easily become unstable and fall into the Sun.)

Facilitator's Note: There are many different misconceptions about gravity; children may think that it is related to an object's motion, its proximity to Earth, its temperature, its magnetic field, or other unrelated concepts. Guide conversations cautiously and listen carefully to what the children say to avoid supporting their misconceptions.

2. Tell the children they will make a model of how objects — like planets — interact in space.

• Have any of the children played on a trampoline? What happens to the surface of the trampoline when you sit on it? What would happen if a friend tried to roll a ball on the surface with you sitting on it?

Explain that space can act much like the surface of the trampoline. The indentations made on the surface represent the "gravity wells" created by massive objects in space.

3. Invite the children to experiment with the same effects on smaller–scale models. Separate the children into groups and give each group a prepared embroidery hoop, suspended in the air on bricks or books. Explain that they will use marbles and Play-Doh balls to model the effects of gravity on objects in space.

• What will happen to the plastic sheets (space) if they add a marble to it? It will stretch out and the marble will roll.
• What will happen if there are two marbles on the sheet? The marbles will roll toward each other.

Facilitator's Note: Gravity is a universal force, like magnetism and electricity. However, it becomes important only at large scales. Gravity determines the interactions stars, planets, and moons.

In the model, the balls are too small to exert a significant gravitational pull on each other. However, they are gravitationally pulled toward Earth! They move toward each other because the weights of heavier objects distort the sheet and lighter objects roll "downhill."

4. Invite the children to experiment with their models of space
by placing and dropping the marbles (together and separately) onto the sheet.

5. Ask the groups to each add a large, 2" round ball of Play-Doh to represent a large "planet" alone on the sheet. Ask the children to hypothesize what will happen if the marbles are dropped onto the sheet, and have them record their thoughts in their journals before they test them. After they have dropped the marbles onto the sheet, share that this "pull" toward the "planets" is a model of gravity.

• How does this model gravity? The marbles are pulled, or "fall," toward the planet.
• Does this large Play-Doh planet represent strong or weak gravity? This planet has strong gravity — the marbles fall straight toward it.

Facilitator's Note: The Play-Doh and Styrofoam balls used in steps 5–7 serve to create test "wells" on the sheets. They should remain stationary while the children roll the marbles to see how they move at each step. Encourage the children to only roll marbles, as the Play-Doh is sticky and will not model the motion accurately.

6. Ask the groups to place a very small round ball of Play-Doh (about half of the size of a marble), which represents a small asteroid, alone on the sheet. Have them note their predictions in their journals and then test what will happen to marbles added to the sheet.

• What will happen if marbles are added to the sheet now? Why? The marbles may take longer to reach the Play-Doh asteroid or may not move toward it at all.
• What type of gravity will a small asteroid have compared to a large planet? It doesn't have very much "gravity".

7. Ask the groups to place the Styrofoam ball alone on the sheet and, keeping records in their journals, experiment with its gravitational pull.

• What type of object might the Styrofoam ball model? It can represent a planet that isn’t very dense, like Saturn.
• How does its size, mass, and density compare to that of the large Play-Doh "planet"? It is about the same size, but less dense and therefore less massive.
• What will happen when the marbles are added? Will they behave more like they did for the large or small Play-Doh planets? Again, the marbles may take longer to reach the low-density giant planet; they won’t feel the pull of gravity as strongly as they did with the very large Play-Doh planet.
• Does Saturn have as much gravity as Jupiter? Saturn’s gravity is not very strong compared to Jupiter’s.

Remind the children that the gravitational pull of a planet depends on its mass and size. Saturn is large in size, but it does not have nearly as much mass packed into its volume as Jupiter does.

Facilitator's Note: Saturn does have plenty of mass, and as they explored in Heavyweight Champion: Jupiter!, it does have gravity. However, because it is not dense, a person standing in its cloud tops would only weigh about as much as they weigh on Earth. Saturn's cloud tops are far above the planet's bulky — and gravitationally strong — center.Because the force of gravity depends on both mass and distance, planets that are puffy and less dense have less gravity at their cloud-tops or surfaces, which are far above the bulk of the mass in their interiors. This is why planets like Saturn appear to have less gravity than Neptune, despite Saturn's greater mass. You may need to remind the children of what they learned in Dunking the Planets in order for them to understand these difficult concepts.

8. Invite the groups to experiment with dropping the marbles in different locations, and with different amounts of Play-Doh or the Styrofoam ball, in various locations on their gravity field.

• Do the marbles ever briefly circle the planet?
• Do they ever avoid the planet?
• Do small asteroids experience gravity? Asteroids and other small bodies, like comets, are also kept in orbit around the Sun by the Sun's large gravitational pull — even when they are at great distances from the Sun. They can also be pulled into orbit around a planet — like Mars' two moons — or impact a moon or planet.

9. After the children have finished experimenting, discuss their findings.

• How did the marbles behave toward the largest Play-Doh planet? They rolled directly toward it. How was this like gravity? The large planet had a lot of mass, and, in our model, a lot of gravity.
• How did the marbles behave with the Styrofoam planet? They may have ignored it completely. Why? The ball did not have much mass, and so it had very little gravity in this model.
• Does a large object always have a lot of mass? No!
• If we can measure the gravity of a planet, and its size, what can that tell us about that planet? The gravitational pull of the planet can tell us more about that planet's mass, which helps us to determine its density and what its interior is like.

Ask the children to draw in their journals, based on their models, how deep a gravity well the Moon, Earth, and Jupiter each create in space. Have them describe how their differences in gravity relate to each object's size and mass.

10. Invite the children to describe how this model of gravity resembles real gravity and how it fails.

• Do objects in the solar system move toward each other with real gravity, like they did in the model? Yes.
• Do objects roll toward each other in space because of gravity? No, they are tugged but they don't roll.
• Do planets in our solar system usually run into each other? No, they are very far apart and they are orbiting the Sun. Sometimes comets and asteroids collide with planets, though.

Facilitator's Note: Children also may not understand that the planets are not being significantly pulled toward each other. They are strongly pulled toward the Sun, but since they are also moving, they move around the Sun in stable orbits. Smaller objects like comets and asteroids may have less circular orbits that cross the paths of planets — sometimes resulting in a collision. Be careful when identifying the objects in this activity not to introduce misconceptions regarding planets' orbits and collisions.

### Conclusion

Explain that the Juno mission to Jupiter will experience Jupiter's gravity in much the same way as a very, very small marble might in our model. Show a picture or video animation of Juno orbiting Jupiter. (Juno will orbit Jupiter, however, rather than falling into it.) Juno's instruments will keep careful track of how Jupiter's pull on the spacecraft changes as the spacecraft passes over the planet's surface. In this way, Juno will be able to measure how Jupiter's gravity is different from place to place. By measuring the slight changes in Juno's trajectory, scientists will learn where exactly Jupiter keeps the bulk of its mass in its deep interior. Scientists can then infer details about the composition of Jupiter's unseen lower layers and core.

• How strong a pull will Juno feel as it orbits Jupiter? A very strong pull!

If possible, build on the children's knowledge by offering them a future Jupiter's Family Secrets activity. Invite the children to return to wrap-up their investigations of Jupiter by attending the concluding activity, My Trip to Jupiter, where they create scrapbooks to document their own journeys into Jupiter's deepest mysteries!