[quote]
First of all, thanks for spending time in this.
I'm not a native English speaker (from the Netherlands) so I hope you understand what I write.
There are some things where must be paid attention to:
If using step, the black curve is not drawn.
If you take R between 1,5 and 1,7 there is something strange going on( ore a new physical law?  Wink)
The same happens between 2 and 2,5 while increasing the radius.
If the curve is good again and you decrease the radius it stays good fore a while.
Checking normal force doesn't change anything in displaying to me.
[/quote]
The above bugs are fixed now.

[quote]
The first part could be a straight line. It mustn't be a curve.
[/quote]
If it is a straight, then there is a dis-continuity when it move into the second part.
Normally, coster are curve path, is not it?

[quote]
The grey square, must it not be hidden if it has no meaning?
[/quote]
I added it to show components of gravity (tangential/normal components)

[quote]
Further requests:
Could the speed of playing the applet made variable, even reverse the motion?
[/quote]
I added another slider to change fps(frame per second), reduce it to smalleer value will show down the motion.
[quote]
Could be switched between play and step during the performing of the motion?
[/quote]
I do not understand what do you mean?
The step button should be useful when the simulation is in paused mode.

[quote]
The kinetic and potential energy you display in the top of the screen, could it be represented as bars?
I don't know what would be better. 2 separate bars  or two bars connected. You can see than that the total amount of energy doesn't change.
[/quote]
I added two connected bars.
[quote]
Could the radius be given in numbers? It is hard to give the exact radius with mouse. 2.00->1.99
[/quote]
I add marks, I hope it is better now.

[quote]
Now the applet represents a not attached object in the looping as I asked. Could it be made to switch to ad attached mode, like a roller-coaster. It won't get a bullet curve if to less energy, but goes back.
[/quote]
If you can provide an image, I will add it as an option.
[quote]
Could the buttons reset and init switched? I press automatic reset but want init.

I hope I'm not demand to much in this feedback.

[/quote]

It is updated! I hope you like the new version.