Drawing an arc tangent to a line

Question

Thanks to a guy in this forum I was able to handle several triangles I had to draw. Now a smaller problem. I did the triangle in the picture, but I just miss the arch tangent to the point. Every time I adjust the center, it moves and I had to start from the beginning. I just need the arc and the letter omega and D2.

Here's what I've done so far:

\begin{figure}[h]
  \centering
  \begin{tikzpicture}[>=latex]

    \coordinate (A) at (0,0);
    \coordinate (B) at (7,0);
    \coordinate (C) at (5,-4);
    \coordinate (ABmid) at (A -| C);
    \coordinate (D) at (0,-4);
    \coordinate (E) at (10,-4);
    \coordinate (F) at (2,0);

    \draw[->] [thick] (A)--(B) node[above,midway]{$U_2$};
    \draw[->] [thick] (C)--(A) node[left,midway]{$W_2$};
    \draw[->] [thick] (C)--(B) node[right,midway]{$C_2$};
    \draw [thick,dashed] (D)--(E);
    \draw[->] [thick] ($(B)+(0,5pt)$)--($(ABmid) +(0,5pt)$) node[above,pos=0.5]{$C_{\theta2}$};
    \draw[thick,dashed] (C)--(F);
    \draw[->] [thick] (C)--(ABmid) node[left,pos=0.7]{$C_{a2}$};

    \pic["$\beta_{2b}$",draw, angle eccentricity=1.2,angle radius=15mm] {angle = F--C--D};
    \pic["$\beta_2$",draw, angle eccentricity=1.2,angle radius=25mm] {angle = A--C--D}; 
    \pic["$\alpha_2$",draw, angle eccentricity=1.2,angle radius=12mm] {angle = E--C--B};

    \draw[thick] (0,-9) arc (180:0:5);

  \end{tikzpicture}
  \caption{\textit{Triangoli di velocità all'estremità della girante}}
  \label{fig:Triangoli girante}
\end{figure}

Answer 1

Like this?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc,angles,quotes} 
\begin{document}
\begin{figure}[h]
  \centering
  \begin{tikzpicture}[>=latex]

    \coordinate (A) at (0,0);
    \coordinate (B) at (7,0);
    \coordinate (C) at (5,-4);
    \coordinate (ABmid) at (A -| C);
    \coordinate (D) at (0,-4);
    \coordinate (E) at (10,-4);
    \coordinate (F) at (2,0);

    \draw[->] [thick] (A)--(B) node[above,midway]{$U_2$};
    \draw[->] [thick] (C)--(A) node[left,midway]{$W_2$};
    \draw[->] [thick] (C)--(B) node[right,midway]{$C_2$};
    \draw [thick,dashed] (D)--(E);
    \draw[->] [thick] ($(B)+(0,5pt)$)--($(ABmid) +(0,5pt)$) node[above,pos=0.5]{$C_{\theta2}$};
    \draw[thick,dashed] (C)--(F);
    \draw[->] [thick] (C)--(ABmid) node[left,pos=0.7]{$C_{a2}$};

    \pic["$\beta_{2b}$",draw, angle eccentricity=1.2,angle radius=15mm] {angle = F--C--D};
    \pic["$\beta_2$",draw, angle eccentricity=1.2,angle radius=25mm] {angle = A--C--D}; 
    \pic["$\alpha_2$",draw, angle eccentricity=1.2,angle radius=12mm] {angle = E--C--B};

    \draw[thick] ($(C)+(60:5)-(90:5)$) arc[start angle=60,end angle=120,radius=5]
    coordinate[pos=0.25] (p);
    \draw[<-] (p) -- ++ (-180+75:1) node[anchor=75]{$D_2$};
    \draw[<-] ([yshift=-0.5cm]C) arc[start angle=90,end angle=110,radius=4.5]
    node[midway,below=1ex]{$\omega$};

  \end{tikzpicture}
  \caption{\textit{Triangoli di velocit\`a all'estremit\`a della girante.}}
  \label{fig:Triangoli girante}
\end{figure}
\end{document}

For the future, I'd kindly ask you to post complete minimal working examples since otherwise only those who know by looking at the code which libraries are needed can answer the question immediately.