fb_contact_reactions()

[nName, tName, tDir]: fb_contact_reactions(o, n, i)

• o the meclib object list (stackjson_parse(objects))
• n the names array (content of the names input field)
• i index of the "contact" object

Source

fb_contact_reactions() returns the names the student gave the reactions at a contact, sorted by direction:

• nName name of the force along the contact normal (the normal force),
• tName name of the force along the tangent (the friction force),
• tDir direction of the friction force as drawn (unit vector), e.g. to read its sense.

Each is false if no such force is found (also while the contact is not deactivated). If there are several, the first one is taken. The tolerance is that of Feedback for "contact" (sine or cosine of the angle below 0.05). Moments and oblique forces are ignored.

It gives no feedback. Check the contact itself with fb_contact() and the names with fb_unique_names(). The function is meant for reference solutions that use the student's names, e.g. equilibrium equations, when several names are accepted for a reaction.

Example: equilibrium in x with the student's names

Taken from demo_questions/contact_demo_03 static friction.xml (smooth contact A, rough contact B, tAB points from A to B).

Feedback variables:

obj: stackjson_parse(objects);
[nA, tA, dA]: fb_contact_reactions(obj, names, i_A);
[nB, tB, dB]: fb_contact_reactions(obj, names, i_B);
if member(false, [nA, nB, tB]) then (
  txt: "Vervollständigen Sie zuerst den Freischnitt.", ok: false
) else (
  sR: signum(float(dB . tAB)),      /* sense of the friction force as drawn */
  ref: subst([N_A = nA, N_B = nB, R_B = tB], N_B*sin(alpha) - sR*R_B*cos(alpha)),
  ok: same_sign(S_GGx, ref),
  txt: if ok then "" else fb_vars(S_GGx, ref, [nA, nB, tB, G])
);

The reference is written with fixed names (N_A, N_B, R_B) and the student's names are substituted.

See also fb_reaction_name() (the name of the single reaction at a support).