This is a simulation for a particle(ball) move on top of a curved surface with z=a (x[sup]2[/sup]+y[sup]2[/sup]).
The red arrow is the velocity for the particle and black arrow is the normal force.
You can change the viewing angle with mouse (click and drag).

VIEW or DOWNLOAD ejs xml source
then click to translate strings for selected locale.

Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop.