Expert: Jesse Martinez - 12/9/2009

Question
Hi Mr. Martinez

I'm confused about Newton’s law of universal gravitation and orbits in relation to Kepler's law.

Newton said that "every object in the universe attracts every other object along a line of the centres of the objects, proportional to each object's mass, and inversely proportional to the square of the distance between the objects."

in his law of universal gravitation Newton states that the gravitational force between any two objects depends on the mass of each object and the distance between them. The greater each object's mass, the stronger the pull, but the greater the distance between them, the weaker the pull. The strength of the gravitational force, in turn, directly affects the speed and shape of an object's orbit. As strength increases, so does the orbital speed and the tightness of the orbit.

Although I've read about the apple and moon experiments, etc., but I still don't understand the elliptical orbit theory (I think that is what it's called).   

Believe it or not I learn something new every day (i.e., the different plashes of the moon, etc.)  The challenge is in interpretating what I've learned.  Hopefully you can assist.

Antoniece

Answer
Kepler formlulated laws about the orbits of planets. First law, they are ellipses. I believe this is what you are referring to. All three laws are stated in the introduction of the first link below. According to those laws. The motions of the planets around the sun and the orbit of an object around another one can be predicted. The two sites below have very useful visual explanations of how these laws work. I always find that visuals are the best way to explain and examine orbital motion.

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/site... Third Law Interactive

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/site... Second Law Interactive

This next site is a little different and allows you to start an object in in orbit by "tossing" it. It is more fun to play with and also allows you to see the "equal areas in equal time" law, the second one that says a line from the sun to the planet with sweep out an equal amount of areas in a given time period.

http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/kepler6.htm

I hope you have fun on these sites and I'm sure you will learn something. You will see direct evidence that the mass of the sun plays a big part in the orbital periods, as does initial velocity and distance. I must note that the mass of the planet is also a factor, but is negligible for these examples, since the sun is much heavier than any object here. The planets actually revolve around the center of gravity of the sun and the planet in question. This is called the barycenter of the two objects. In all solar system cases, the barycenter of the sun and any one planet lies within the sun itself.