MATLAB: Driven Damped Pendulum Axes Question

equation of motionnewbiependulum

Hello, I am quite new to MATLab, this is my very first program, which is also a uni assignment. This is my program thus far:
m=2;
l=5;
g=9.81;
c=0.5;
theta=45;
theta_dot=5;
t=0;
dt=(0.1/theta_dot);
while t<50
    theta_dotdot=-(g/l)*sin(theta) - (c/m*l^2)*theta_dot; %motion eqn
    theta=theta+dt*theta_dot;
    theta_dot=theta_dot+dt*theta_dotdot;
    figure(1);
    axis([0 0 -2*pi -2*pi]);
    hold on;
    plot(theta_dotdot, theta);
    t=t+dt;            
end
What I have done so far (the code above explained): 1.Giving some values (the first part of the code). 2.The equation of motion (%motion eqn). 3.Theta and Theta_dot increasing in value (recommended method/code I was given in the assignment). 4. Figure – doesn't work, and I am not quite sure how it should work. 5. Plot – no reason for it not to work, I assume it doesn't plot anything because of the figure error above. 6. t increasing. 7. Everything above is looped, with t=t+dt What I need to do: Firstly, I need it to give me a figure of the pendulum positions (don't know how to explain this better)- the figure part of the code Secondly, I need it to plot theta_dotdot, theta. I think I got this part right, but the program does not run that part yet, im assuming because of the errors in the figure part.
There are the erros im getting:
>> run Untitled2
Error using set
Bad property value found.
Object Name: axes
Property Name: 'XLim'
Values must be increasing and non-NaN.
Error in axis>LocSetLimits (line 201)
    set(ax,...
Error in axis (line 93)
                LocSetLimits(ax(j),cur_arg);
Error in Untitled2 (line 17)
    axis([0 0 -2*pi -2*pi]);
Error in run (line 63)
evalin('caller', [script ';']);
>>
If an answer can not be provided, I could really use some help on the errors, why am I getting them, what do they mean.

Best Answer

The implementation is acceptable, the first remark is that visualization commands of results should be outside the loop, otherwise you obtain N plots where N is the number of iterations. While loop in this case can be changed into for loop but not necessary, the following version is working , but the physical interpretation still needs correction of the code, try to adjust it :
   m=2;
l=5;
g=9.81;
c=0.5;
thet=45;
dtheta=5;
t=0;
dt=(0.1/dtheta);
cc=1;
for t=1:dt:50
    d2theta(cc)=-(g/l)*sin(thet) - (c/m*l^2)*dtheta; %motion eqn
    theta(cc)=thet+dt*dtheta;
    dtheta=dtheta+dt*d2theta(cc);
    cc=cc+1;            
end
    figure(1);
%    axis([0 0 -2*pi -2*pi]);
 %   hold on;
    plot(d2theta, theta);
Related Question