;D You can do what you want with that applet. ::)
If you did not change the default setting in the applet: [b]Fix kinetic energy[/b] when change initial conditions.
You will get planet motion with the same eccentricity of ellips.

However, if you change it to "[b]fix angular momentum[/b]", then you will get  planet motion with different eccentricity of ellips.  :-*

That is why I add the following sentences in the page:
[quote]
You can drag the blue arrow to change its initial condition in three different modes:
1. fixed kinetic energy
2. fixed angular momentum
3. arbitrary
Find out the relation between shape and size of the trajectory with the above parameters.
[/quote]

I hope students can really play with it and discover the difference , and figure out some relations!  ;)