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Helping asked for Trochoïd and Frenet Coordinates

Posted 1 year ago

I am trying to get a left trochoid (R=1, d=2) of a left trochoid of a left trochoid. To do so, I try to establish a Frenet system and get a curve derived from the first one, that makes two turn when the first make one and then by the same way, i try to get a second curve that makes four turns when the second makes two and the first makes one.

But I can't get Mathematica do it. I must make a lot of mistakes, as I am not so familiar with Frenet and Mathematica.

I need to obtain equations for C1, C2 and C3 (see graphic attached). I think C1 is a left trochoïd, where R=1 and d=2 for example (elongated trochoÏd), C2 and C3 are the same, only perioding faster , C2 along C1, and C3 along C2 along C1.

Here is the code I made so far (see notebook)

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POSTED BY: Cédric Mannu
3 Replies

I am not sure what you want to do. Keep in mind that matrix multiplication is done with . and not with *

This is my interpretation of your wish:

f0[t_] = {x[t, 1, 2], y[t, 1, 2], z[t, 1, 2]};
f1[t_] = {x2[t_], y2[t_], z2[t_]} =
   f0[t] +
    T1[t] x[2 t, 1, 2] +
    N1[t] y[2 t, 1, 2] +
    B1[t] z[2 t, 1, 2];
curves = 
  ParametricPlot3D[{f0[t], f1[t]}, {t, -4 Pi, 4 Pi}, PlotRange -> All]
Manipulate[
 Show[curves, Graphics3D[{{PointSize[Large], Blue, Point[f0[t]],
     Brown, Point[f1[t]]}, HalfLine[f0[t], T1[t]],
    Arrow[{f0[t], f0[t] + 15 T1[t]}], HalfLine[f0[t], N1[t]],
    Arrow[{f0[t], f0[t] + 15 N1[t]}], HalfLine[f0[t], B1[t]],
    Arrow[{f0[t], f0[t] + 15 B1[t]}]}]],
 {t, -4 Pi, 4 Pi}]
POSTED BY: Gianluca Gorni
Posted 1 year ago

It helps for visualization. Yes, I made first mistakes with matrix multiplication and i corrected since.

My wish is to have a function like f0 rolling around f0 (C2 rolling around C1 in the graphic), thus the frenet system. My second curvature, to calculate, is like f0 but in frenet system of the initial f0, name it C2 in the graphic, same equation than C1, but in C2 system (frenet system).

Then I want to do the same a third time, having C3 (same curvature as always but in C2 frenet system now). And I want to have C3 formal equation.

I tried the code you propose and get a point going along C1, which means the Frenet system is ok. But I don't get visualization of C2, and the system complains about some values.

So much thanks for helping the poor lad I am,

POSTED BY: Cédric Mannu

Try with

f1[t_] = {x2[t_], y2[t_], z2[t_]} = f0[t] +
    N1[t] Cos[2 t] + B1[t] Sin[2 t];
POSTED BY: Gianluca Gorni
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