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# Circling: What Keeps a Satellite in Orbit Around a Planet?

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Source:
Author: Janice VanCleave

### Problem

What keeps a satellite in orbit around a planet?

### Materials

• cookie sheet with raised sides
• cardboard tube from a toilet tissue roll
• sheet of typing paper
• cup
• tap water
• red food coloring
• spoon
• marble
• modeling clay

### Procedure

1. Lay the cookie sheet on a table.
2. Place the cardboard tube in one corner of the cookie sheet so that one end of the tube rests on the rim of one short side of the pan.
3. Secure the raised end of the tube to the rim of the pan with tape.
4. Lay the paper in the pan so that the untaped end of the tube rests on the edge of the paper.
5. Fill the cup about one-fourth full with water and add 10 drops of food coloring. Stir.
6. Wet the marble with the colored water, place it in the elevated end of the tube, and release it.
7. Prop up the long side of the pan nearest the tube about 1 inch (2.5 cm) by placing 2 lumps of clay under both corners of the long side.
8. Again, wet the marble, place it in the tube, and release it.

### Results

Spots of red water mark the two paths of the marble. The path across the level pan is straight, while the path across the raised pan is curved.

### Why?

Gravity (the force pulling things toward the center of the earth) pulls the marble down the elevated tube and holds the marble on the flat pan as it rolls forward in a straight line. On the elevated pan, the marble moves forward as it leaves the raised tube, but the earth's gravity tries to pull the marble down toward the lower end of the pan. The combination of the marble's forward motion and the downward pull of gravity causes the marble to move in a curved path. The path of a satellite (a celestial body that revolves about another celestial body), like that of the marble on the elevated pan, curves because its forward speed and the planet's gravity pull it down toward the planet. Without gravity, satellites would move in a straight path, and without forward speed, gravity would pull the satellite into the planet. The curved path that a satellite traces around a planet is called an orbit.

### Let's Explore

1. How would an increase in forward speed affect the orbit of a satellite? Repeat the experiment, using a different sheet of paper and a lump of clay to further raise the elevated end of the tube.
2. How would an increase in gravity affect the path of a satellite? While gravity does not change greatly around a given planet, the results of increasing the pan's elevation can be used to simulate the effect on satellites orbiting different planets with stronger gravity. Repeat the original experiment, using a different sheet of paper and more clay to increase the elevation of the pan.
3. With an increase in gravity, would an increase in forward speed send the marble on the same curved path as in the original experiment? On a different sheet of paper, trace the curved path made by the marble in the original experiment, and place the tracing in the pan. Repeat the previous experiment, increasing the height of the elevated end of the tube. Science Fair Hint: Use the papers with red water-marked paths from each experiment as part of a science fair display.
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