Drawing an arc

[Dan]

Hello!  I'm having troubles with drawing an arc.  The rest of the diagram
works fine and the code seems to parse but nothing shows up on the picture.

Here's the code:
\begin{tikzpicture}[domain=3:10]
     \draw (0,0) rectangle (3,5);
     \draw (0,0) -- (10, 0);
     \draw [<->] (2,0) -- (2,5);
     \coordinate [label=left:{$h$}] (h) at (2,2.5);
     \draw [<->] (3,-0.5) -- (10,-0.5);
     \coordinate [label=below:{$R$}] (R) at(6.5,-0.5);
     \draw (3,5) -- (5,5);
     \draw[->] (3,5) -- (5,7);
     \path (3,5) to node [above] {$v_0$} (5,8);
     \path (3,5) arc [start angle=0, end angle=45, radius=3mm];
     \coordinate [label={$\theta$}] (t) at (4, 5.1);
     \draw [dashed] plot
(\x,{5+tan(45)*((\x)-3)-4.9*(((\x)-3)/(6.33*cos(45)))^2});
\end{tikzpicture}

The problem is the path line 4 lines above the bottom.  This is practically
a copy of a line I got out of a manual but I can't seem to make it work.
 The arc is supposed to appear in the corner next to the symbol theta.  See
the attachment.  (It's a little fuzzy, but it will do.)

Thanks!

-Dan
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