Applet illustrates the first and second laws of Kepler.
You could imagine that the orange object shown here is a satellite going round the earth ( shown in blue ). You can vary the eccentricity of its orbit and observe the motion. You should be able to notice the change in the velocity vector as the satellite goes around the earth.
The graph on the right is plot of kinetic energy (red), potential Energy (blue) and total energy (yellow) versus time. You should be able to notice that total energy is a constant. You should also be able to notice that when e=0, the orbit is circular and all energies are constant and kinetic energy is half of the modulus of potential energy.
Kepler's Third Law