# simulation of spherical pendulum

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Answered: Bas Siebers on 26 Feb 2015
First, pleasue excuse my bad in English. I am from Germany and trying my best.
In the long run I want to simulate a spherical double pendulum. But it became obvious that I already have problems with simulating a single spherical pendulum.
Here is the code I use. The equations should ber right so far. It should run, when pasted into matlab:
%===================================================================
function Erg1 = solveitmass();
options = odeset('Mass' , @mass, 'RelTol',1e-8,'AbsTol',1e-8);
Erg1 = ode45(@rightside, [0 10], [pi/4 0 0 0.01], options);
end
%============================Function for Massmatrix
function M = mass( t, x)
m = 1;
l = 1;
M = [1 0 0 0;
0 1 0 0;
0 0 m*l^2 0;
0 0 0 m*l^2*sin(x(1))^2]
end
%===========================Function for right hand side of equations
function dx = rightside(t, x)
l = 1;
g = 9.81;
m= 1;
dx = zeros(4, 1);
dx(1)=x(3)
dx(2)=x(4)
dx(3)=+m*l^2*sin(x(1))*cos(x(1))*x(4)^2-m*g*l*sin(x(1))
dx(4)= -2*m*l^2*sin(x(1))*cos(x(1))*x(3)*x(4)
end
%======================================================
The position of the mass is decribed by two angles. I will refer to them as azimuth-angle and elevation-angle. the first 2 positions of the initial condtion vector are the angles (Elevation first, second Azimuth). The last two positions describe the the angular rates. And here lies the problem. When starting with the angles with arbitrary values and the angular rates = 0, there ist no problem (with these values one does basicallyhave a 2D-Pendulum). But as soon as I give a value to the angular rate of the azimuth angle, the elevation angle doesn't cross zero anymore. I attached a plot, that shows the values of the elevation-angle.
Can anybody tell me what I am doing wrong? The equations should be right and as far as I can see, the implementation is also correct.
Edit
To me this doen't seem to be the natural behaviour of a pendulum. In my opinion the elevation angle should follow the course of a sinus wave. I can understand that the the angular rate of the azimuth angle gets bigger when the elevation gets smaller because the conversation of angular momentum, but I don't see a reason that the elevation cannot cross zero anymore, as soon as there is any movement in azimuth direction in the initial conditions. Instead the elevation angle really 'bounces' back again as soon as ist reaches zero. And I have no idea why this happens. I see no reason why the elevation angle just crosses zero. Since the Equations of movement are correct I was hoping that I am just using Matlab in some wrong way.

#### 1 Comment

myta on 2 Apr 2013