LaTeX forum ⇒ Graphics, Figures & Tables ⇒ tikZ | Coordinate Parameters

[psionman]

I am using Pgf Tikz to produce a diagram

It shows a mass at rest on a ramp. For most points I have been able to parameterise the coordinates (meaning that, for example, I can change the angle of the ramp or the mass size by changing a single parameter. The code follows:

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\documentclass[a4paper,12pt]{article}
\usepackage{tikz}
\usetikzlibrary {shapes}
 
%Mass on ramp
 
\newcommand{\base}{14}
\newcommand{\massXposition}{6}
\newcommand{\massSide}{2}
\newcommand{\massHalf}{1}
\newcommand{\incline}{20}
 
\newcommand{\massVertex}{(intersection of \massXposition,0--\massXposition,1 and 0,0--\incline:1)}
\newcommand{\massCentre}{[rotate around={\incline:\massVertex}] \massVertex +(\massHalf,\massHalf)}
 
\begin{document}
\begin{tikzpicture}[scale=.5]
 
%Ramp
\draw  (0,0) -- (\incline:\base) -- (\incline:\base |- 0,0) -- cycle; 
 
%Mass
\draw [rotate around={\incline:\massVertex}] \massVertex rectangle +(\massSide,\massSide);
\fill \massCentre circle (2pt);
\draw [->] \massCentre --  +(-1,1);
\draw [->] \massCentre --  +(1,3);
\draw [->] \massCentre -- (5.85,6 |- 0,0);
%\draw [->] \massCentre -- (\massCentre |- 0,0);
\end{tikzpicture}
\end{document}

I now want to add an arrow vertically downwards from the centre of the mass. The line

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\draw [->] \massCentre -- (5.85,6 |- 0,0);

gives me this, but I have had to 'hard-code' the x-coordinate of the mass Centre. If I use

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\draw [->] \massCentre -- (\massCentre |- 0,0);

I get an error

Package tikz Error: Cannot parse this coordinate.

Also can someone explain how the relative coordinates in

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\draw [->] \massCentre --  +(-1,1);

work? I would have expected

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\draw [->] \massCentre --  +(-2,2);

[Stefan Kottwitz]

Hi,

welcome to the board!

Have a look at Free body diagrams in TikZ, there's also a mass on a ramp, and arrows on a mass object.

Stefan

[psionman]

Thanks Stefan

I'll study with interest