Kepler's Laws

PREVIOUS LESSON                                                                                                                              NEXT LESSON
In a rush?
Need a quick review?
The following video is a very brief summary.
Otherwise keep moving
Between 1609 and 1619, Johannes Kepler published his planetary laws of motion. This was based on the data he received from his mentor,  Tycho Brahe.  As he studied the data he discovered that the planets revolved around the sun,  not in circular orbits as many believed,  but in elliptical orbits. He also noted that the speed at which the planets moved varied depending on the distance away from the sun.
As a result, in his publication, he developed three laws that describe the behaviour of planets.

Interactive
We will start by looking at some animations that demonstrate what these three laws are about.
  1. Click on the First Law and press PLAY
    The first law states that planets move in elliptical orbits
    Ellipses are shapes that have two foci (click FOCI) and have an associated eccentricity
    Eccentricity is a measure of how elliptical it is, and the greater the eccentricity (closer to 1) the more eccentric it is
    (Perfect circles have an eccentricity of 0)
    The Sun, in this case is at one of the foci
  2. Click on the Second Law and press PLAY
    In essence Kepler's second law states that a satellite sweeps out equal areas in the same time frame. The result is that the satellite moves faster  when closer to the central body, and slower when further away.
​We will leave the third law animation till later
Video
Now watch the video on Kepler's Laws. The first 5 mins or so lay the historical background. If you wish to go straight to the three laws, scroll to 5:38
 

Now revisit the animation above and revise the first and second law. 
Examine the Third Law.
Press play and see where it plots on the graph.
Alter the planets' position and press play again, and repeat this a number of times.
You will see that the ratio of T2/a3 will be the only graph that will produce a straight line, that is, T2/a3 is a constant.
This confirms Kepler's Third Law.

 Check your understanding
Sample Problem
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
Picture
 
 
PREVIOUS LESSON                                                                                                                              NEXT LESSON