Please download the following Interactive Physics Files which illustrate orbital motion under the influence of a gravitational force:

• Orbit 0.sit.hqx Shows a circular orbit with a r^-2 force
• Orbit 1.sit.hqx Shows an elliptical orbit with a r^-2 force
• Orbit 2.sit.hqx and Orbit 3.sit.hqx The force in no longer a r^-2 force. Note how the forces were set up in the Force Field.. under the World menu. You might want to see how the Force Field was set up for Orbit0 and Orbit1.
• Orbit SS.sit.hqx This shows the Sun, Mercury and Earth. You can go to the Sun, Mercury or Earth refernece frames. From the Earth, you can observe the retrograde motion of Mercury.