Straight line junction with an arc

Question

The following is a minimal working example for what I'm trying to achieve. I need to create a figure that looks like an U, that is, a capital U. However the junction between each of the straight lines and the arc is "sloppy". Is there a way to improve that?

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\draw[very thick,blue] (0,10) -- (0,0);
\draw[very thick, blue] (0,0) arc (-180:0:3);
\draw[very thick, blue] (6,0) -- (6,10);
\end{tikzpicture}

\end{document}

Answer 1

A quick way is to use the "to" operator for lines: replace your code by

\begin{tikzpicture}
\draw[very thick,blue] (0,10) -- (0,0) to[out=270,in=180] (3,-3) to [out=0,in=270] (6,0) -- (6,10);
\end{tikzpicture} 

This way you can specify the angles or directions that lines leave or approach a point.

Answer 2

I don't think there is anything wrong with the image, just your PDF viewer. Here is an image using Acrobat to view it zoomed in 1600%, where I have added the red circle around the center of where the lines join:

However, you could draw it all in one operation as per Frédéric's answer, or by rewriting your code as:

\draw[very thick,blue] (0,10) -- (0,0) arc (-180:0:3) -- (6,10);

Code:

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
  \draw[very thick, red ] (0,0) circle (0.5cm);
  \draw[very thick, blue] (0,10) -- (0,0);
  \draw[very thick, blue] (0,0) arc (-180:0:3);
  \draw[very thick, blue] (6,0) -- (6,10);
\end{tikzpicture}

\begin{tikzpicture}
\draw[very thick, red] (0,0) circle (0.5cm);

\draw[line width=6pt,blue] (0,10) -- (0,0) arc (-180:0:3) -- (6,10);
\end{tikzpicture}

\end{document}