If you want to have something falling down under gravity.
Just add a gravity as an downward acceleration. ie. Fd= -m*g; // where m is the mass, g is the gravity
To make it more realistic, you can add upward drag force which is proportional to downward speed. i.e. Fu=-b*v;// where b is a constant.
So the total force is F=-m*g-b*v;
The above are physics involved. The rest are up to you how to present in a visual way.
I do not want to download those files.
Because it is a risk to download some executable file form somewhere you did not know and run it in your computer.