Etydhv

I'm working in a project for university using latex and i'll make a gif explaning the radians of math! And I really dont know how to make this type of line (i'll put the example below) !! Plz help me guys!!

The project that ive done so far :D
http://www.mediafire.com/file/wc4xazt8k4h4t5h/testeAnime.pdf/file

WHAT IVE DONE :D

documentclass[border=1cm]standalone
usepackage[utf8]inputenc
usepackagetikz
begindocument

begintikzpicture





beginscope
draw (0,0) circle (10cm);
draw[line width=0.15cm, blue] (10,0) arc (0:360:10) ;

draw[-] (-12,0) -- (12,0) node[right] $x$;
draw[-] (0,-12) -- (0,12) node[above] $y$;
draw[line width=0.15cm,red] (0,0) -- (10,-0);
node[scale=15.5, red, right, opacity=0.5] at (2,2) r;

draw[red][fill] (0,0) circle (0.2cm);
draw[red][fill] (10,-0) circle (0.2cm);


 endscope


endtikzpicture

enddocument

A PSTricks solution only for fun purposes.

documentclass[pstricks,border=12pt,12pt]standalone
radians
defobj#1%
beginpspicture(-4,-4)(4,4)
 pscircle4
 pssetlinecolor=red,linewidth=2pslinewidth
 psarc(0,0)40#1
 rput#1(4;#1)psline(!0 1 #1 sub 4 mul)
endpspicture
begindocument
 foreach i in 0,0.1,...,1.0obji
enddocument

If you wish to produce such animated gifs, you may want to compile

documentclass[border=1cm,tikz]standalone
usetikzlibraryarrows.meta
begindocument
foreach X in 180,175,...,90
begintikzpicture
beginscope
draw (0,0) circle (10cm);
draw[line width=0.15cm, blue] (10,0) arc (0:360:10) ;
draw[-] (-12,0) -- (12,0) node[right] $x$;
draw[-] (0,-12) -- (0,12) node[above] $y$;
draw[line width=0.15cm,red,Circle-Circle,shorten >=-2*pgflinewidth,shorten
<=-2*pgflinewidth] (10,0) -- ++(X:10);
node[scale=15.5, red, right, opacity=0.5] at (2,2) r;
endscope
endtikzpicture
enddocument

with pdflatex and then convert the resulting pdf with

convert -density 300 -delay 34 -loop 0 -alpha remove outfile.pdf ani.gif

to get

Or

documentclass[border=1cm,tikz]standalone
usetikzlibraryarrows.meta
begindocument
foreach X in 0,5,...,90
begintikzpicture
draw[line width=0.15cm, blue] circle[radius=10cm];
draw[-] (-12,0) -- (12,0) node[right] $x$;
draw[-] (0,-12) -- (0,12) node[above] $y$;
draw[line width=0.15cm,red,Circle-Circle,shorten >=-2*pgflinewidth,shorten
<=-2*pgflinewidth] (0,0) -- ++(-X:10)
node[scale=15.5, red, midway,above,sloped,opacity=0.5] at (2,2) r;
endtikzpicture
enddocument

Or, inspired by this answer

documentclass[border=1cm,tikz]standalone
usetikzlibraryarrows.meta
begindocument
foreach X in 0,5,...,90
begintikzpicture
draw[line width=0.15cm, blue] circle[radius=10cm];
draw[-] (-12,0) -- (12,0) node[right] $x$;
draw[-] (0,-12) -- (0,12) node[above] $y$;
draw[line width=0.15cm,red,Circle-Circle,shorten >=-2*pgflinewidth,shorten
<=-2*pgflinewidth] (10,0) arc(0:2*X/pi:10) -- ++ (2*X/pi+90:10-10*X/90);
endtikzpicture
enddocument