You have the following code at Evolution Page:
[code]
dv/dt=-k*y2/m-9.8
[/code]
However, y2 is re-calculated at "Fixed Relation" Page,
which is processed after  "Evolution Page".

[code]
if (y < 0 ) y2 = y;
else y2 = 0;
[/code]

At "Custom page": Click to create a page
You will find default code like the following
[code]
public void lib_Page () {
}
[/code]

Change it to
[code]
public double myYvalue(double y){
if(y<0)return y;
return 0;
}
[/code]

Then in the "Evolution Page"
[code]
dv/dt = -k*myYvalue(y)/m-9.8
[/code]

You can change myYvalue to any name you want, however, you need to use the same name in the evolution page.

The function myYvalue will be called in the evolution page (at the time it is needed).