The mass of the bodies affects the force of gravity. The sun is a giant mass of burning plasma. It is so massive that it exerts a very strong gravitational field on the planets, forcing them to orbit around the sun. However, that is only part of the explanation. What else affects the strength of gravity and the orbit of the planets?

Let’s use the same interactive to explore this question.

 Click to Run

Source: My Solar System. PhET Interactive Simulations, University of Colorado at Boulder

Directions:

1. Copy the following table into your notes.
2.  Position of Planet   (body 2) Time of One Orbit (seconds) 50 75 100 125 150 175 200 225
3. Select the following options on the simulation.
1. From the Select Preset pull down choose Sun and planet.
2. Make sure the following are checked: System Centered, Show Traces, Show Grid
3. Move the green slider all the way to accurate.

The simulation should look like the one below if you have selected the correct options.

4. In this exercise you will leave the masses of the sun and planet as they are. The only thing you will be changing is the position (x) of the planet (body 2). You will not be changing the y value.
5. Change the position of the planet by entering the first planet position into the x value. You should see that planet moved closer to the sun.
6. Press the start button and let the planet (body 2) complete one orbit around the sun. Press stop when it returns back to the starting point.
7. Record the time in seconds it took the planet to complete one orbit in the data table.
8. Click the reset button.
9. Repeat steps 4-7 to complete the data table. Then answer the following questions in your notes.

Questions:

1. When is the planet orbit the fastest?
2. What makes the planet orbit time increase or decrease?

1. When the distance of the planet was set at 50 (In other words, when it was closest to the sun.)
2. The distance the bodies are from each other