Sums of Forces and Moments - mkraska/meclib GitHub Wiki

Sum of of forces and moments along the standard coordinate axes

[fx,fy,m]: sum_xyM(obj,names,rp,lu) Returns the sums of forces fx, fy and moments m about reference point rp using length unit lu.

  • objects Input field with object list
  • names Input field with object names
  • rp coordinates of the reference point in grid units [x,y]
  • lu symbolic length unit

Sum of forces along a custom direction

Use the "dir" object to let the user specify custom coordinate directions for sums of forces.

sum_dir(o, n, d)

  • o list of objects
  • n names field
  • d direction vector, e.g. direction(o[i_dir])

Returns the sum of all "force" objects in o, projected onto direction d, as a symbolic linear combination of their names n[i] — this is the grading target for an equation of equilibrium in a student-chosen direction.

Question Variables

Make sure that fb_fbd.mac is included. It contains the relevant functions.

Feedback Variables

This is a typical use if the reference point is obtained from the position of object i_P, which could be a "crosshair"

obj: stackjson_parse(objects);
[fx,fy,m]: sum_xyM(obj,names,names[i_P],a);

PRT Node

Display of the sum of forces and moment (formative feedback).

<p>\( \displaystyle \Sigma F_x={@fx@}\) </p>
<p>\( \displaystyle \Sigma F_y={@fx@}\) </p>
<p>\( \displaystyle \Sigma M_0={@m@}\) </p>