I am making changes to the initialization page
// to make the button initialise do the same thing as reset thanks to tips
t = 0;
// bring carts to start position
x1 = -0.8 ;
x2 = 0 ;

but i realize the plot trace draw a line back to (t,momentum) = (0,0)

when do i add the line make the trace clear ?

And suddenly, i remember Christain Wolfgang codes.
i just understood how to do this by using on initialisation page.


cool, i solve it myself ?!
when typing then i realised, so just post it for others to read. ;)