Hello, I have been searching for forever to try and figure out this Kepler's law stuff. I know that planets don't follow circular paths around the sun, but rather their paths are eliptical, the same with moons and other things. My question is "Why?" The diagram that I looked at showed that one of the foci would be the sun (for a planet orbiting the sun) and the other would be some blank place in space, so what I want to know is why is that blank place in space so important that planets are attrated to it. The same goes for moons orbiting the planets. Any explainations or links on the topic would be greatly appreciated. Thanks a lot for all of your help.

Originally posted by Evlich

Hello, I have been searching for forever to try and figure out this Kepler's law stuff. I know that planets don't follow circular paths around the sun, but rather their paths are eliptical, the same with moons and other things. My question is "Why?"

 

Actually, the real question would be, "Why would you expect the orbit to be circular?" In order to get a perfectly circular orbit, the initial conditions have to be so precise as to make it highly unlikely to ever happen in practice. So, most orbits are elliptical.

 

The diagram that I looked at showed that one of the foci would be the sun (for a planet orbiting the sun) and the other would be some blank place in space, so what I want to know is why is that blank place in space so important that planets are attrated to it.

 

Planets aren't attracted to it. They are attracted to the sun and follow elliptical orbits. The definition of an ellipse is a curve that is such that the sum of the distances from each of two foci to any point on the curve is constant. In the case of the Earth's orbit around the Sun, one focus is at the attracting center and the other focus just happens to not have a body located at it.

In brief...Kepler's Laws are as follows:

 

1. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

 

2. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.

 

In others words, the planet moves fastest when it is near perihelion and slowest when it is near aphelion (furthest from the sun.)

 

3. The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes.

 

In other words, period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Mercury, the innermost planet, takes only 88 days to orbit the Sun but the outermost planet (Pluto) requires 248 years to do the same.

so what I want to know is why is that blank place in space so important that planets are attrated to it.

 

it's the same effect (although without orbit) as letting a basket-ball bounce on the ground without touching it while it's bounching.

 

Then you can also ask yourself why is it attracted to that 1meter high point above the ground. obviously it isn't.

 

You may want to look up potential and kinetic energy.

Just like the basketball, a pendulum is similar. It's velocity is greatest at the very bottom of its motion, and the momoentum caries untill the pull of gravity is no longer compensated for. Also very good example for potential and kinetic examples, and lots of rough test wuestions.