Newton's Aquarium 1.1
Newton's Aquarium is a physics program that helps you see Newton's Law of Universal Gravitation in action so you can better understand how it works.
You can create your own solar systems to apply the law to, or view the ones in the program package. Zooming in on the orbiting planets will help you to get a better feel for this law.
There are questions included with the solar systems that are available in the demo. Hopefully, these questions will get you into thinking about Newton's law in relation to how these planets orbit. Enjoy.