Gravity and Jupiter

Newton's First Law of Motion states that an object at rest will tend to stay at rest and an object in motion will tend to stay in motion in a straight line, unless acted upon by an external, unbalanced force. If one rolls a ball on a level surface, it will travel in a straight line until the force of friction stops it. There's no air or carpeting to slow the planets in their orbits, but the planets don't travel in straight lines through space; they travel around in nearly circular orbits. There must be another force acting upon them. That other force is gravity. When we experience gravity in our everyday lives, i.e.; why we don't float around, the force is usually between two objects of very different masses: one very massive (like the Earth) and another with negligible mass (like a person). Because of this fact, we often simply think of gravity as some property of very massive objects like planets and stars. In reality, though, gravity is a characteristic of mass. All mass has gravity, and the amount of gravity is governed by the amount of mass. Thus, any mass can feel all other masses because of the common fact that any mass has gravity; however, the combined gravity will govern the amount of tug on the masses. So, eventhough the Earth is enormous compared to you, the Earth pulls on you only as much as you pull on the Earth. Since the Earth is much more massive than you, though, you won't notice the infinitesimal amount the Earth moves towards you, but you can't help but notice how you fall to Earth! There is also gravitational attraction between you and, say, your desk, but neither you nor your desk have enough mass to make that force noticeable. The gravitational force between two bowling balls can be measured in the laboratory with very sensitive instruments, however!

In the formation of the solar system, every object formed with a speed in some direction; however, every object also has an amount of mass equal to the amount of stuff inside of them. For example, Jupiter's mass tugs on the mass of its moons like the mass of the Earth tugs on us and keeps us on the ground. So why don't the moons just fall on to Jupiter and stick there? The gravity of Jupiter pulls the moons towards it but each of its moons is trying to speed past it in some direction. As Jupiter tugs inward, the moon has its own speed sideways -- the combination is the moons orbit around Jupiter in nearly circular orbits.

If the Sun is so incredibly massive, why should anything want to orbit around a less massive planet like Jupiter? Gravity is very powerful, but is actually limited: you will feel it less and less the further away from the mass you get.

Since gravity's force decreases as you move further away from the source of mass, a little moon which is born really close to a planet will orbit that planet, eventhough the planet is less massive than the Sun.

The students can understand this with a simple activity.

ACTIVITY 2: Gravity and Orbit Simulation

• 1. Make a frame of pencils along the edges of the back of the Jupiter drawing. Rest the drawing face up on its frame, slipping a bar magnet underneath the sheet so that it is directly under Jupiter and oriented vertically.
• 2. Sprinkle iron filings lightly on top of the drawing. Continue until you notice a pattern appearing. Try to get a good sprinkling across the entire paper. What can you see? Draw the pattern on a clean printout or carefully run your pencil along the pattern in the filings.

DISCUSSION:

What can you see in the pattern? How extensive is this pattern and where are the moons in all of this? What about the other planets?

Students should discover that the pattern reaches well into the orbit of Saturn! The most intense portions of the pattern are in the area of space where the little Galilean moons orbit. What implications does this have for these moons?

Io, since it is volcanic, is often effusing particles into space. These particles get caught up along the magnetic field lines and travel to the atmosphere of Jupiter. There, these particles disturb the atmosphere which in turn gives off radio waves. The final and most complex activity involves the building of a radio telescope to pick up these emissions.

ACTIVITY 3: The Jovian Basketball Hoop

MATERIALS:short-wave radio, 66 inches of stiff copper wire, a length of coaxial cable, 2ft square of metal screen or chicken wire, dowels enough to make 4, 12-inch lengths, wood saw, knife, and tape.

• 1. (Please refer to the drawing) Bend the stiff wire around a school garbage can, making an unconnected loop.
• 2. Carefully cut the dowels into 4 lengths of 12-inches each. Tape these vertically at set points around the wire loop.
• 3. Turn the loop over and tape the free end of each stick to the square of screen/chicken wire. The loop should now sit on top of the 12 inch dowels which are taped to the screen base.
• 4. With a knife, most carefully slit open one end of coaxial cable so that you can see the different layers inside. Peel off the plastic covering and pull back the metal foil. DO NOT TEAR OFF THE FOIL! Twist the foil into a long strand and bend to the side. Cut off the remaining plastic to get to the copper wire inside. Connect this copper wire to one free end of the loop and connect the foil twist to the screen.
• 5. Expose the other free end of the coaxial cable and attach this to the antenna base of the short-wave radio. In other words, your Jovian Hoop now functions as the radio's new antenna.
• 6. Using an almanac or sky calendar, locate the position of Jupiter in the day or night sky. Although the Jovian Hoop does not need to be pointed too accurately at Jupiter, it is best to prop the Jovian Hoop on to an easel or on blocks, books, etc, until it roughly points to Jupiter's location.
• 7. Turn on the radio and tune to the area of 18-22 MHz. The radiation you will hear should sound like waves crashing on a beach. The white noise of unclear stations may confuse you, but the Jovian decametric radiation will be distinguishable because of the surging quality of the sound. Do not be discouraged if it takes several tries to receive the emissions. Inspect your set-up carefully and try later.
• 8. (Optional, great for Science Fairs) Students may wonder if there is ever a difference in the sound through time. The answer is yes, and they should consider recording the sound during different times of the week. Recall that material from Io rushes up to Jupiter's atmosphere all of the time. Proper timing of the recordings with the passing of Io between the Earth's line of sight and Jupiter will provide interesting results to those patient enough to make continued measurements. (Recall also that Io orbits Jupiter in just under 2 days...observations made carefully over a few days will provide interesting results. Students may want to record the emissions for further and more careful analysis. The earphone jack of the radio can be connected to a tape recorder or reel-to-reel machine for recordings.

DISCUSSION:

What have students learned about astronomical observing? Is it possible to make detailed observations with even the crudest of instruments? Were students surprised to be able to make observations in the daytime? Why can the Jovian Hoop "see" Jupiter but our eyes cannot? Hint: why is the sky blue? Which was more fun -- making the Jovian Hoop, predicting the results, or listening to Jupiter? Do answers to this question show students that astronomy can be engineering, theory, and observation?

By Jove, I Think We've Got It!: Jupiter Resources

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