# Can You Throw a Football to Pluto?

Your challenge will be to demonstrate how large the solar system is by modeling it outdoors. Each person will take a turn acting as the sun, accepting the football challenge ... can you throw the football all the way to Pluto? Make it a contest between your friends!

Materials:

• At least ten friends or classmates
• 10 stakes or something similar to stick into the ground to mark the distance between the planets.
• Ruler or meter stick
• Football

Procedure:
1. Have the person representing the sun stand in one place and then begin to measure, in a straight line, the distance from the sun to each planet.
2. Use the information from the chart below for your measurements.
• (The chart shows the distances from the sun to each planet in a unit called Astronomical Units [AU]; one AU is the distance from the sun to Earth.
3. To make this model as accurate as possible, have one AU equal to one meter.
 Planet Approximate Distance from Sun (AU) Mercury 0.3 Venus 0.7 Earth 1.0 Mars 1.5 Jupiter 5.2 Saturn 9.5 Uranus 19.8 Neptune 30.6 Pluto 39.5
1. Place a stake in the ground where each planet will be in your scaled down solar system.
2. Have your friends, who are representing planets, line up in at each one of the stakes.
3. Now for the challenge!
• The person representing the sun is to throw a football to Pluto.
• Rotate each person in one planet (with the person representing Mercury going to the sun position and the person who was the sun going to the Pluto position.)
• Repeat the challenge
4. The person who gets the football to the farthest planet wins.

Analysis:

1. How far away from the sun is Pluto?
2. What was the farthest planet to which each person was able to throw the football?
3. Why was it difficult to throw the football beyond Pluto, the last planet in the solar system?
4. Alpha Centauri is he closest star outside of our solar system. If you were to try to throw a football to this star, you would need to throw it over 273 kilometers (164 miles). How many Astronomical Units is that?

So what keeps the Universe together?
If the solar system is so large, what keeps it all together? You already read about how the heat of the early sun caused the differences between the inner and outer planets. However, what keeps those planets in orbit around the sun? Two primary phenomena act together to keep the planets in orbit.

Thanks to the work of Isaac Newton in the 1600's, we have a much better understanding of how the solar system works. Newton is well-known for developing his three laws of motion, one of which states that an object in motion tends to stay in motion at the same velocity unless acted on by an outside force. The characteristic of an object to do that is called inertia. You experience this when you are in a car and go around a corner. Your body tries to keep moving in the same direction it was, but the seat belt or side of the car makes you change your direction.

Inertia is only one of the significant reasons for the solar system shape. There must be some sort of outside force causing the planets to change direction instead of traveling in a straight line. That force is called gravity. You are probably familiar with the story of Newton watching an apple fall from the apple tree and developing what is called the Law of Universal Gravitation. He stated that every object in the universe is attracted to all other objects. The strength of that attraction depends on the mass of the objects and the distance between them. This gravitational attraction is what opposes the inertia of the planets and keeps them from flying off into space in a straight line.

An easy way to illustrate this is to tie a ball to the end of a string or a rope. Twirl the ball above your head in a circle. Experiment with different lengths of rope between you and the ball. Watch the QuickTime video to see an example.

Analysis:

1. Why does the solar system look the way it does?
2. Describe at least two ways the inner planets are different from the outer planets?
• What caused those differences?
3. Why is Newton credited with developing the Law of Universal Gravitation?
4. What relationship does inertia play in keeping a planet in orbit?
5. Describe the role of gravity in Earth's revolution around the sun?
6. In the video example, what does the rope attached to the ball represent?
• What would happen if the ball were released?
• How does this example model the motion of the planets?

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Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE Earth Systems Science core.

Updated October 24, 2008 by: Glen Westbroek