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LaTeX 中的振動系統與 TikZ：將現有的擺移動到特定位置

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LaTeX 中的振動系統與 TikZ：將現有的擺移動到特定位置

我是 Tikz 的新手，我設法組合幾個 Tikz 程式碼範例 [x] 來表示 3 自由度系統。但是，我想“翻譯”/移動質量中的鐘擺中號到第二質量M2但它不起作用。 pdf內容的中心似乎被定義在M中心以及角度的「定義」。當我嘗試移動鐘擺時，我沒有實現改變，角度不會轉換為下一個質量 M2。

請參閱下面的程式碼：

\documentclass[tikz]{standalone}
\usetikzlibrary{calc,patterns,decorations.pathmorphing,decorations.markings}

\usepackage{physics}
\usepackage{siunitx}
\sisetup{detect-all} % to allow \pi in \SI
\usepackage{tikz,pgfplots}
\usepackage[outline]{contour} % glow around text
\usetikzlibrary{calc}
\usetikzlibrary{angles,quotes} % for pic
\usetikzlibrary{arrows.meta}
\tikzset{>=latex} % for LaTeX arrow head
\contourlength{1.2pt}

\colorlet{xcol}{blue!70!black}
\colorlet{vcol}{green!60!black}
\colorlet{myred}{red!70!black}
\colorlet{myblue}{blue!70!black}
\colorlet{mygreen}{green!70!black}
\colorlet{mydarkred}{myred!70!black}
\colorlet{mydarkblue}{myblue!60!black}
\colorlet{mydarkgreen}{mygreen!60!black}
\colorlet{acol}{red!50!blue!80!black!80}
\tikzstyle{CM}=[red!40!black,fill=red!80!black!80]
\tikzstyle{xline}=[xcol,thick,smooth]
\tikzstyle{mass}=[line width=0.6,red!30!black,fill=red!40!black!10,rounded corners=1,top color=red!40!black!20,bottom color=red!40!black!10,shading angle=20]
\tikzstyle{faded mass}=[dashed,linewidth=0.1,red!30!black!40,fill=red!40!black!10,rounded corners=1,top color=red!40!black!10,bottom color=red!40!black!10,shading angle=20]
\tikzstyle{rope}=[black!70!black,thick,line cap=round]
\def\rope#1{ \draw[black,line width=0.8] #1; \draw[rope,line width=1.1] #1; }
\tikzstyle{force}=[->,myred,very thick,line cap=round]
\tikzstyle{velocity}=[->,vcol,very thick,line cap=round]
\tikzstyle{Fproj}=[force,myred!40]
\tikzstyle{myarr}=[-{Latex[length=3,width=2]},ultra thick]
\def\tick#1#2{\draw[thick] (#1)++(#2:0.12) --++ (#2-180:0.24)}
\DeclareMathOperator{\sn}{sn}
\DeclareMathOperator{\cn}{cn}
\DeclareMathOperator{\dn}{dn}
\def\N{80} % number of samples in plots

\tikzstyle{axis}=[->,thick] %line width=0.6

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\tikzstyle{spring}=[thick,decorate,decoration={zigzag,pre length=0.3cm,post length=0.3cm,segment length=6,amplitude=1.75mm}] %amplitude=1.75mm
\tikzstyle{damper}=[thick,decoration={markings,  
  mark connection node=dmp,
  mark=at position 0.5 with 
  {
    \node (dmp) [thick,inner sep=0pt,transform shape,rotate=-90,minimum width=15pt,minimum height=3pt,draw=none] {};
    \draw [thick] ($(dmp.north east)+(2pt,0)$) -- (dmp.south east) -- (dmp.south west) -- ($(dmp.north west)+(2pt,0)$);
    \draw [thick] ($(dmp.north)+(0,-5pt)$) -- ($(dmp.north)+(0,5pt)$);
  }
}, decorate]
\tikzstyle{ground}=[fill,pattern=north east lines,draw=none,minimum width=0.75cm,minimum height=0.3cm,inner sep=0pt,outer sep=0pt]



\definecolor{monOrange}{RGB}{255,157,0}
\definecolor{monBleu}{rgb}{0.2,0.4,0.6}
\definecolor{monCyan}{RGB}{74,181,247}
\definecolor{monGris}{RGB}{100,100,100}
\definecolor{monRed}{RGB}{255, 36, 0}

\tikzset{
    seisme/.style={color=cyan,ultra thick,->},
        vent/.style={color=monRed,ultra thick,->},
    }   

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}


%%%%%%%%%%%%%%% SHEMA MODAL 3DDL %%%%%%%%%%%%%%%
\begin{tikzpicture}[scale=1.1, every node/.style={scale=1.3}]
% Rectangle masse m1 + petites roues
\node [draw, outer sep=0pt, thick] (M) [minimum width=2cm, minimum height=1.5cm] {$M$};
\draw [thick, fill=white] (M.south west) ++(0.2cm,-0.125cm) circle (0.125cm)  (M.south east) ++(-0.2cm,-0.125cm) circle (0.125cm);

% Rectangle masse m2 + petites roues
\node [draw, outer sep=0pt, thick] (M2) [minimum width=2cm, minimum height=1.5cm, xshift = 4cm] {$M_2$}; % rectangle entourant la masse m2
\draw [thick, fill=white] (M2.south west) ++(0.2cm,-0.125cm) circle (0.125cm)  (M2.south east) ++(-0.2cm,-0.125cm) circle (0.125cm); % 2 petits cercles sous la masse m2

% Ground bas
\node (ground) [ground,anchor=north,yshift=-0.2cm,minimum width=10cm,xshift=2.03cm] at (M.south) {}; % ground bas hachurage
\draw (ground.north west) -- (ground.north east) -- (ground.south east) -- (ground.south west); %traits horizontaux entourant le ground bas
\node (fill) [ground,xshift=-0.15cm,minimum height = 0.3cm, minimum width = 0.3cm] at (ground.west) {}; %petit ajout de ground bas tout à gauche
\draw (fill.north west) -- (fill.south west) -- (fill.south east);

% Ground vertical
\node (wall) [ground, rotate=-90, minimum width=3cm,anchor=south east] at (fill.north west) {}; % ground vertical hachurage
\draw (wall.north east) -- (wall.north west) -- (wall.south west) -- (wall.south east);  %traits horizontaux entourant le ground vertical

% % Ressort k
\draw [spring] (M.east) -- (M2.west) node (k) [midway,yshift=0.4cm] {$k$};

% Ressort k
\draw [spring] (wall.15) node[left= 0.3cm]{$O$} -- ($(M.north west)!(wall.15)!(M.south west)$) node [midway,yshift=0.4cm] {$k$};

% Force d'excitation sur la masse m2
%\draw [-latex,ultra thick] (M2.east) -- +(1cm,0cm) node [right] (u) {$F(t) = F_0 \sin{\omega t}$};

%Déplacement de la masse m1 noté y1
\draw [-latex,ultra thick] (M.center) ++(0cm, 1.4cm) -- +(1cm,0cm) node [above] (y1) {$x(t)$}; % flèche déplacement x(t)
\draw [dashed] (M.center) -- +(0cm,1.4cm); % trait pointillés y1

% Déplacement de la masse m2 noté y2
\draw [-latex,ultra thick] (M2.center) ++(0cm, 1.4cm) -- +(1cm,0cm) node [above] (y2) {$x_2(t)$}; % flèche déplacement x(t)
\draw [dashed] (M2.center) -- +(0cm,1.4cm); % trait pointillés y2

% Axes (x,y) et centre 0 du repère cartésien
%\draw[axis] (0,0)--(5.7,0) node[right] {$\overrightarrow{x}$}
\draw[dashed,->](M2)--(7.7,0) node[right]{$\overrightarrow{x}$};
\draw[dashed,->](wall.15)--(-3.5,5) node[left]{$\overrightarrow{y}$};

% PENDULUM
\def\L{5}  % string length
\def\ang{28} % angle string
\def\R{0.4} % ball radius
\def\F{1.0}  % force magnitude
  \coordinate (M) at (\ang-90:\L);
  \coordinate (M') at (0,-\L);
  \coordinate (O) at (0,0);
  \coordinate (B) at (0,-\L-2.2*\R);
  \coordinate (FT) at ($(M)+(90+\ang:{\F*cos(\ang)+\R})$);
  \coordinate (FG) at ($(M)+(-90:{\F+\R})$);
  \coordinate (FGx) at ($(M)+(-90+\ang:{0.55*\F+\R})$);
  \coordinate (MA) at ($(M)+(180+\ang:{\F*sin(\ang)+\R})$);
  %\draw[faded mass] (M') circle(\R);
  \draw[dashed] (O) -- (B);
  \draw[dashed] (-90+\ang+10:\L) arc(-90+\ang+10:-110:\L) (B);
  \rope{(O) -- (M)} \path (O) -- (M) node[midway,above right=-1] {$L$};
  \fill[black] (O) circle(0.04);
  \draw[mass] (M) circle(\R) node {$m$};
  \draw pic[myarr,"$\theta$",xcol,draw=xcol,angle radius=43,angle eccentricity=1.30] {angle=B--O--M};


\end{tikzpicture}



\end{document}

這是我得到的： 三自由度系統

歡迎任何建議！

Pendulum TikZ 代碼源碼

地面-質量-彈簧系統代碼來源

答案1

為了好玩，動畫模擬結果。

運動方程式是透過系統的動能和位能導出的拉格朗日形式主義並透過\pstODEsolve包裝解決pst-ode。 在此輸入影像描述 在此輸入影像描述

%\documentclass[margin=3pt,10pt,export]{standalone} % for GIF
%\documentclass[margin=3pt,10pt,dvisvgm]{standalone} % for SVG, run twice: dvilualatex example ; dvisvgm --font-format=woff --zoom=-1 --page=1 example.dvi
\documentclass[margin=3pt,10pt]{standalone} % for PDF, run lualatex twice
 
\usepackage{pst-ode}
\usepackage[loop,controls]{animate} 
\usepackage{tikz} 
\usetikzlibrary{calc,arrows.meta,decorations.pathmorphing,patterns.meta}
\usepackage{listofitems} % read space separated items 
\usepackage[T1]{fontenc} 
\usepackage{xsavebox} 
 
% adjustable parameters & definitions 
\pstVerb{
tx@Dict begin % all definitions within PSTricks dict 
  /tEnd 60 def      % time span to be simulated [s]
%  /tEnd 13.56 def   % this time span gives a nearly cyclic impression, suitable for GIF
  /m1 1 def         % mass_1 [kg] 
  /m2 1 def         % mass_2 [kg] 
  /m3 0.1 def       % mass_3 (bob) [kg]
  /L 2 def          % pendulum length [m] 
  /k1 80 def        % spring constant [kg/s^2] 
  /k2 40 def        % spring constant [kg/s^2] 
  /g 9.81 def       % acceleration [m/s^2] 
  /x2Zero 0.9 def   % initial displacement of mass_2 [m] 
  /thetaZero 0 def  % initial displacement of pendulum from vertical [°] 
  %  
  % (integer) number of time steps (for 25 frames per s) + 1 
  /N (cvi(tEnd*25+1)) AlgParser cvx exec def
  %
  % initial displacement of mass_1
  /x1Zero (k2/(k1+k2)*x2Zero) AlgParser cvx exec def
  %
  % some constants 
  /k12M1 ((k1+k2)/m1) AlgParser cvx exec def
  /k2M1  (k2/m1)      AlgParser cvx exec def
  %
  % displacements of mass_1, mass_2, pendulum
  /x1    (x[0]) AlgParser cvx def
  /x2    (x[1]) AlgParser cvx def
  /theta (x[2]) AlgParser cvx def
  %
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  % 1st order ODE system (equations of motion) 
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  % velocities
  /v1    (x[3]) AlgParser cvx def
  /v2    (x[4]) AlgParser cvx def
  /omega (x[5]) AlgParser cvx def
  % accelerations
  /a1    (k2M1*x2 - k12M1*x1) AlgParser cvx def
  /a2    ((m3*sin(theta)*(g*cos(theta)+L*omega^2)-k2*(x2-x1))/(m2+m3*sin(theta)^2)) AlgParser cvx def
  /alpha (-(a2*cos(theta) + g*sin(theta))/L) AlgParser cvx def
end
}
 
% solve equations of motion 
\pstODEsolve[algebraicAll,saveData]{timeX1X2Theta}{    % PS variable and file (base-)name that take result list 
  t | x1 | x2 | RadtoDeg(theta-TwoPi*cvi(theta/TwoPi)) % table format of data to be saved in timeX1X2Theta(.dat)
}{0}{tEnd}{N}{                                         % t_0, t_end, number of  time steps + 1 
  x1Zero | x2Zero | DegtoRad(thetaZero) | 0 | 0 | 0    % initial conditions (displacements, velocities)
}{ 
  v1 | v2 | omega | a1 | a2 | alpha                    % ODE system's RHS 
} 
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
% \fileopenr{<file stream>}{<file name>}, opens file for reading 
\newcommand\fileopenr[2]{% 
  \newread#1% 
  \immediate\openin#1=#2% 
} 
% \readtolist[<sep char>]{<file stream>}{\list} 
% reads a line from file stream and splits at <sep char> into \list[1], \list[2], ... 
\newcommand\readtolist[3][,]{{% 
  \setsepchar{#1}% 
  \immediate\read#2 to \inputline% 
    \ifeof#2 
      \immediate\closein#2% 
      \ifdefined\multiframebreak\multiframebreak\fi% 
    \else% 
      \greadlist*#3\inputline% 
    \fi% 
}} 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

\tikzset{spring/.style={decorate, decoration={zigzag,pre length=0.2cm,post length=0.2cm,segment length=\dimexpr(#1-0.4cm-1pt)/14\relax, amplitude=0.1cm}}}

\begin{document}
\IfFileExists{timeX1X2Theta.dat}{}{dummy\end{document}}%
% cart
\begin{xlrbox}{cart}
  \begin{tikzpicture}[line width=0.2mm]
    \useasboundingbox (-1,0) rectangle (1,1.6);
    \draw (-0.99,0.1) rectangle (0.99,1.59);
    \fill [radius=0.1] (0,0.8) -- ++(0.1,0) arc [radius=0.1,start angle=0,end angle=90] -- ++(0,-0.2) arc [start angle=270, end angle=180];
    \draw [thin] (0,0.8) circle [radius=0.1];
    \filldraw [fill=white] (-0.8,0.1) circle [radius=0.08] (0.8,0.1) circle [radius=0.08];
  \end{tikzpicture}
\end{xlrbox}%
%
\begin{animateinline}{25}
  \fileopenr{\data}{timeX1X2Theta.dat}%
  \readtolist[ ]{\data}{\table}%
  \multiframe{10000}{}{
    \begin{tikzpicture}[line width=0.2mm]
      \useasboundingbox (-0.3,-1.5) rectangle (10.1,3.1);
      % ground & wall
      \draw [pattern={Lines[angle=-45,line width=0.2pt,distance={3pt/sqrt(2)}]}] (0,0) -- (10,0) -- (10,-0.25) -- (-0.25,-0.25) -- (-0.25,2) -- (0,2) --cycle;  
      % cart 1
      \path [shift={(\table[2],0)}] (3,0.8) node[inner sep=0, name={cart1}] {\thecart} node [anchor=south] {$m_1$};
      % mass_2 (cart2) and mass_3 (bob) centres
      \path[shift={(\table[3],0)}] (7,0.8) coordinate (mass2) ++(\fpeval{\table[4]-90.0}:2) coordinate (bob);
      % cart 2
      \path (mass2) node[inner sep=0] (cart2) {\thecart} node [anchor=south] {$m_2$};
      % springs
      \coordinate (wall) at (0,0.8);
      \draw[line join=round, line cap=rect] let \p1=($(cart1.west)-(wall)$) in [draw,spring={\x1}] (wall) -- (cart1.west) node [midway,above,yshift=1pt] {$k_1$};
      \draw[line join=round, line cap=rect] let \p1=($(cart2.west)-(cart1.east)$) in [draw,spring={\x1}] (cart1.east) -- (cart2.west) node [midway,above,yshift=1pt] {$k_2$};
      % pendulum
      \draw (mass2) -- (bob) node[inner sep=1pt,midway,anchor=west,rotate={\table[4]}] {$L$};
      \draw [thin,fill=white] (mass2) circle [radius=0.03];
      \draw [thin,fill=white!90!black] (bob) circle [radius=0.21];
      \path (bob) node[rotate={\table[4]}] {\scriptsize $m_3$};
      % coordinates
      \draw[very thin,dashdotted] (3,0.8) -- ++(0,1); \draw[-latex] (3,0.8) ++(0,1) -- ++(0.5,0) node[above] {$x_1$};
      \draw[very thin,dashdotted] (7,0.8) -- ++(0,1); \draw[-latex] (7,0.8) ++(0,1) -- ++(0.5,0) node[above] {$x_2$};
      \draw[very thin,dashdotted] (mass2) -- ++(0,-0.55);
      \draw[-latex] (mass2) ++(0,-0.5) arc [radius=0.5,start angle=270,end angle=300] -- ++(30:0.1) node[inner sep=0pt,above right] {\small $\theta$};
    \end{tikzpicture}%
    \readtolist[ ]{\data}{\table}
  }
\end{animateinline}
\end{document}

答案2

最簡單的方法可能是僅shift通過 進行擺碼(M2)。

移動擺

我部分清理、最小化和更新了您的程式碼，並修復了它以避免錯誤。

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,patterns,angles,quotes,arrows.meta,decorations.pathmorphing}
\tikzset{>=latex} % for LaTeX arrow head

\colorlet{xcol}{blue!70!black}

\tikzset{mass/.style={line width=0.6,red!30!black,fill=red!40!black!10,rounded corners=1,top color=red!40!black!20,bottom color=red!40!black!10,shading angle=20}}
\tikzset{rope/.style={black!70!black,thick,line cap=round}}
\tikzset{force/.style={->,myred,very thick,line cap=round}}
\tikzset{myarr/.style={->,ultra thick}}

\newcommand\rope[1]{ \draw[black,line width=0.8] #1; \draw[rope,line width=1.1] #1; }

\tikzset{spring/.style={thick,decorate,decoration={zigzag,pre length=0.3cm,post length=0.3cm,segment length=6,amplitude=1.75mm}}, %amplitude=1.75mm
  damper/.style={thick,decoration={markings,  
      mark connection node=dmp,
      mark=at position 0.5 with 
      {
        \node (dmp) [thick,inner sep=0pt,transform shape,rotate=-90,minimum width=15pt,minimum height=3pt,draw=none] {};
        \draw [thick] ($(dmp.north east)+(2pt,0)$) -- (dmp.south east) -- (dmp.south west) -- ($(dmp.north west)+(2pt,0)$);
        \draw [thick] ($(dmp.north)+(0,-5pt)$) -- ($(dmp.north)+(0,5pt)$);
      }
    }, decorate},
  ground/.style={fill,pattern=north east lines,draw=none,minimum width=0.75cm,minimum height=0.3cm,inner sep=0pt,outer sep=0pt},
}   

\begin{document}
%%%%%%%%%%%%%%% SHEMA MODAL 3DDL %%%%%%%%%%%%%%%
\begin{tikzpicture}[scale=1.1, every node/.style={scale=1.3}]
  % Rectangle masse m1 + petites roues
  \node [draw, outer sep=0pt, thick] (M) [minimum width=2cm, minimum height=1.5cm] {$M$};
  \draw [thick, fill=white] (M.south west) ++(0.2cm,-0.125cm) circle (0.125cm)  (M.south east) ++(-0.2cm,-0.125cm) circle (0.125cm);
  
  % Rectangle masse m2 + petites roues
  \node [draw, outer sep=0pt, thick] (M2) [minimum width=2cm, minimum height=1.5cm, xshift = 4cm] {$M_2$}; % rectangle entourant la masse m2
  \draw [thick, fill=white] (M2.south west) ++(0.2cm,-0.125cm) circle (0.125cm)  (M2.south east) ++(-0.2cm,-0.125cm) circle (0.125cm); % 2 petits cercles sous la masse m2
  
  % Ground bas
  \node (ground) [ground,anchor=north,yshift=-0.2cm,minimum width=10cm,xshift=2.03cm] at (M.south) {}; % ground bas hachurage
  \draw (ground.north west) -- (ground.north east) -- (ground.south east) -- (ground.south west); %traits horizontaux entourant le ground bas
  \node (fill) [ground,xshift=-0.15cm,minimum height = 0.3cm, minimum width = 0.3cm] at (ground.west) {}; %petit ajout de ground bas tout à gauche
  \draw (fill.north west) -- (fill.south west) -- (fill.south east);
  
  % Ground vertical
  \node (wall) [ground, rotate=-90, minimum width=3cm,anchor=south east] at (fill.north west) {}; % ground vertical hachurage
  \draw (wall.north east) -- (wall.north west) -- (wall.south west) -- (wall.south east);  %traits horizontaux entourant le ground vertical
  
  % % Ressort k
  \draw [spring] (M.east) -- (M2.west) node (k) [midway,yshift=0.4cm] {$k$};
  
  % Ressort k
  \draw [spring] (wall.15) node[left= 0.3cm]{$O$} -- ($(M.north west)!(wall.15)!(M.south west)$) node [midway,yshift=0.4cm] {$k$};
  
  % Force d'excitation sur la masse m2
  %\draw [-latex,ultra thick] (M2.east) -- +(1cm,0cm) node [right] (u) {$F(t) = F_0 \sin{\omega t}$};
  
  %Déplacement de la masse m1 noté y1
  \draw [-latex,ultra thick] (M.center) ++(0cm, 1.4cm) -- +(1cm,0cm) node [above] (y1) {$x(t)$}; % flèche déplacement x(t)
  \draw [dashed] (M.center) -- +(0cm,1.4cm); % trait pointillés y1
  
  % Déplacement de la masse m2 noté y2
  \draw [-latex,ultra thick] (M2.center) ++(0cm, 1.4cm) -- +(1cm,0cm) node [above] (y2) {$x_2(t)$}; % flèche déplacement x(t)
  \draw [dashed] (M2.center) -- +(0cm,1.4cm); % trait pointillés y2
  
  % Axes (x,y) et centre 0 du repère cartésien
  %\draw[axis] (0,0)--(5.7,0) node[right] {$\overrightarrow{x}$}
  \draw[dashed,->](M2)--(7.7,0) node[right]{$\overrightarrow{x}$};
  \draw[dashed,->](wall.15)--(-3.5,5) node[left]{$\overrightarrow{y}$};
  
  % PENDULUM
  \def\L{5}  % string length
  \def\ang{28} % angle string
  \def\R{0.4} % ball radius
  \def\F{1.0}  % force magnitude
  \begin{scope}[shift=(M2)]
    \coordinate (Mx) at (\ang-90:\L);
    \coordinate (M') at (0,-\L);
    \coordinate (O) at (0,0);
    \coordinate (B) at (0,-\L-2.2*\R);
    \coordinate (FT) at ($(Mx)+(90+\ang:{\F*cos(\ang)+\R})$);
    \coordinate (FG) at ($(Mx)+(-90:{\F+\R})$);
    \coordinate (FGx) at ($(Mx)+(-90+\ang:{0.55*\F+\R})$);
    \coordinate (MA) at ($(Mx)+(180+\ang:{\F*sin(\ang)+\R})$);
    \draw[dashed] (O) -- (B);
    \draw[dashed] (-90+\ang+10:\L) arc(-90+\ang+10:-110:\L) (B);
    \rope{(O) -- (Mx)} \path (O) -- (Mx) node[midway,above right=-1] {$L$};
    \fill[black] (O) circle(0.04);
    \draw[mass] (Mx) circle(\R) node {$m$};
    \draw pic[myarr,"$\theta$",xcol,draw=xcol,angle radius=43,angle eccentricity=1.30] {angle=B--O--Mx};
  \end{scope}
\end{tikzpicture}

\end{document}

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