mjhjmtu

I am making a picture using foreach. MWE is appended below

documentclassarticle

usepackage[svgnames]xcolor
usepackagetikz

begindocument

begintikzpicture

 foreach x in 0,...,5
 draw [DodgerBlue](x,-1) circle (0.25);

foreach x in 0,...,5
 draw [-latex,red](x,-1) -- (x,-1.5);

foreach x in 0,...,5
 draw [-latex,DarkGreen](x,-1) -- (x,-0.5);

draw [ultra thin] (5,-0.5) -- (0,-1);

endtikzpicture

enddocument

The resulting picture is attached below

Now what I want is that the green arrows on the top should be limited by the black line diagonal line. That is for the first circle there should be no green arrow, and its length should increase until it becomes equal to the red arrow in the last circle. How can this be done using foreach using the variable y. Is there a way such that the intersection of the black diagonal line with the green arrows can be used to limit the length of the green arrows to the desired values? Or is there any other way to achieve this?

In case you do not want to do analytic computations (or if you do not have a simple parametrization for the line).

documentclassarticle

usepackage[svgnames]xcolor
usepackagetikz
usetikzlibrarycalc
begindocument

begintikzpicture

 foreach x in 0,...,5
 draw [DodgerBlue](x,-1) circle (0.25);

foreach x in 0,...,5
 draw [-latex,red](x,-1) -- (x,-1.5) ;

draw [ultra thin] (5,-0.5) coordinate(aux0) -- (0,-1) coordinate(aux1);

foreach x in 1,...,5
 draw [-latex,DarkGreen] (x,-1) coordinate(aux2) (x,-0.5) coordinate(aux3)
 (aux2) -- (intersection cs:first line=(aux2)--(aux3), 
 second line=(aux0)--(aux1));


endtikzpicture

enddocument

To make @ArtificialStupidity happy (?) one loop...

documentclass[border=3.14mm]standalone
usepackage[svgnames]xcolor
usepackagetikz
usetikzlibrarycalc
begindocument
begintikzpicture
draw [ultra thin] (5,-0.5) coordinate(aux0) -- (0,-1) coordinate(aux1);
foreach x in 0,...,5
 draw [DodgerBlue](x,-1) circle (0.25);
 draw [-latex,red](x,-1) -- (x,-1.5) ;
 unlessifnumx=0% 
 draw [-latex,DarkGreen] (x,-1) coordinate(aux2) (x,-0.5) coordinate(aux3)
 (aux2) -- (intersection cs:first line=(aux2)--(aux3), 
 second line=(aux0)--(aux1));
 fi
 
endtikzpicture
enddocument

You can use a simple linear function. Please note that your arrow heads are quite large in your scale. It might be a good idea to choose a larger scale or use another arrow tip to have a better representation of the second case.

documentclassarticle

usepackage[svgnames]xcolor
usepackagetikz

begindocument

begintikzpicture

foreach x in 0,...,5
 draw [DodgerBlue](x,0) circle (0.25);
 draw [-latex,red](x,0) -- (x,-.5);

foreach x in 1,...,5
 draw [-latex,DarkGreen](x,0) -- (x,0.1*x);


draw [ultra thin] (5,.5) -- (0,0);

endtikzpicture

enddocument

A PSTricks solution with just one loop only for comparison purposes.

documentclass[pstricks]standalone
usepackagepst-calculate
begindocument
beginpspicture[arrowsize=.2,arrowinset=0,linewidth=1pt](8.5,pscalculate8/3)
 foreach i in 0,1,...,5%
 pstVerb/x ispace 1.5 mul .5 add def%
 pscircle[linecolor=blue](!x 1).5
 psline[linecolor=red]->(!x 1)(!x 0)
 ifnumi=0relaxelsepsline[linecolor=green]->(!x 1)(!x ispace 3 div 1 add)fi
 
 psline[linestyle=dashed](.5,1)(!8 dup 3 div)
endpspicture
enddocument

one more with use of lines intersections, but defined with help of the package intersections:

documentclass[tikz, svgnames, margin=3]standalone
usetikzlibraryintersections

begindocument
 begintikzpicture
 foreach x in 0,...,5
 draw [DodgerBlue](x,0) circle (0.25);
 draw [-latex,red](x,0) -- (x,-.5);

draw [ultra thin, name path=A] (5,.5) -- (0,0);
foreach x in 1,...,5

 path[name path=Bx] (x,0) -- ++ (0,0.5);
 draw[-latex,DarkGreen,
 name intersections=of=A and Bx,by=Bx] (x,0) -- (Bx);

 endtikzpicture
enddocument

Like this (please observe foreach x in 1,...,5 instead of foreach x in 0,...,5?

documentclassarticle

usepackage[svgnames]xcolor
usepackagetikz

begindocument

begintikzpicture

 foreach x in 0,...,5
 draw [DodgerBlue](x,-1) circle (0.25);

foreach x in 0,...,5
 draw [-latex,red](x,-1) -- (x,-1.5);

%foreach x in 0,...,5
 % draw [-latex,DarkGreen](x,-1) -- (x,-0.5);
foreach x in 1,...,5
draw [-latex,DarkGreen](x,-1) -- (x,-1+0.1*x);

draw [ultra thin] (5,-0.5) -- (0,-1);

endtikzpicture

enddocument