Take the 2-minute tour ×
Mathematica Stack Exchange is a question and answer site for users of Mathematica. It's 100% free, no registration required.

So I am completely new to Mathematica, so sorry if this is a dumb question.

I have a set of 3 coupled nonlinear differential equations. They are (a,b,c,D,L constants): $$mx''=Dx'\sqrt{x'^2+y'^2+z'^2}+L(az'-by')$$ $$my''=Dy'\sqrt{x'^2+y'^2+z'^2}+L(bx'-cz')$$ $$mz''=Dz'\sqrt{x'^2+y'^2+z'^2}+L(cy'-ax')$$ And I have initial conditions for $x,y,z,x',y',z'$

Will I be able to solve this for x,y,z? Any help is appreciated!

share|improve this question
    
I am confused on the syntax as well. [eqns, u, {t, tmin, tmax}] is in the documentation. So how do I solve for 3 equations? –  yankeefan11 Dec 5 '13 at 17:42
    
Follow Nasser's lead: write your equations (ODEs and ICs) in terms of x[t],y[t],z[t], and supply them to NDSolve after specifying the values of the constants (you can use a replacement rule like eqs /. {m->1, D->2, L->1, a->.3, b->.2,c->.5} inside NDSolve. Then you will obtain your solution in the form of a list of three interpolating functions. –  Peltio Dec 5 '13 at 17:57

1 Answer 1

up vote 4 down vote accepted

Example:

ClearAll[t, x, y, z];
parms = {d -> 1, L1 -> 10, a -> 5, b -> 99, c -> 8, m -> 100};
term = Sqrt[x'[t]^2 + y'[t]^2 + z'[t]^2];
eq1 = m x''[t] == d x'[t] term + L1 (a z'[t] - b y'[t]);
eq2 = m y''[t] == d y'[t] term + L1 (b z'[t] - c y'[t]);
eq3 = m z''[t] == d z'[t] term + L1 (c z'[t] - a y'[t]);
ic1 = {x'[0] == 0, x[0] == 1};
ic2 = {y'[0] == 2, y[0] == 3};
ic3 = {z'[0] == 0, z[0] == 1};

Now call NDSolve

 sol = NDSolve[{eq1, eq2, eq3, ic1, ic2, ic3} /. parms, {x[t], y[t],z[t]}, {t, 0, 10}]

Mathematica graphics

Now can plot the solution, say

  Plot[Evaluate[x[t] /. sol], {t, 0, 10}]

Mathematica graphics

Or the 3 solutions in one plot (can use legends to label them, etc....)

 Plot[Evaluate[{x[t], y[t], z[t]} /. sol], {t, 0, 10}]

Mathematica graphics

ParameterPlot3D per request

 ParametricPlot3D[Evaluate[{x[t], y[t], z[t]} /. sol], {t, 0, 10}, 
 BoxRatios -> {1, .5, .5}, AxesLabel -> {"x[t]", "y[t]", "z[t]"}]

Mathematica graphics

share|improve this answer
    
A 3D parametric plot would be more appealing :-) –  Peltio Dec 5 '13 at 18:07
    
How would that work? This is for the trajectory of a baseball, so 1 plot would be amazing –  yankeefan11 Dec 5 '13 at 18:28
    
@yankeefan11 Please look up ParametricPlot3D in the docs. –  Szabolcs Dec 5 '13 at 18:29
    
@Peltio, add p3D, is this what you had in mind? –  Nasser Dec 5 '13 at 18:47
    
@Nasser Yep, that's it. Trajectory in 3D space. –  Peltio Dec 5 '13 at 21:16

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.