Chace - in the future, please format your code (as written in your question) so that it is readable. Highlight that text which is code and press the {} Code button. Or, even better, just attach the code to your question using the paperclip button.
Your code will generate undefined x and y error messages since you do not define these variables anywhere in the code. Presumably theta is the angle of the pendulum relative to the y-axis. So with its initial value of 40 degrees, this means that the pendulum is in the lower-right quadrant of the xy plane (beneath the x-axis and to the right of the y-axis). We can determine the (x,y) coordinate for each position of the pendulum (given its angle relative to the y-axis) using the fact that the length of the "string" is one (which will be the hypotenuse of a triangle)
strLength = 1;
x = cosd(90-theta)*strLength;
y = -cosd(theta)*strLength;
If we want to plot the movement of the pendulum, we can do that as
% create the new figure
figure;
% set the limits on the x- and y-axes (add offsets of 0.02)
xlim([min(x)-0.02 max(x)+0.02]);
ylim([min(y)-0.02 0.02]);
hold on;
% draw the initial position of the pendulum as a green circle (go)
plotHandle = plot(x(1),y(1),'go');
% draw the "string" from the origin to the position of the pendulum
lineHandle = line([0 x(1)],[0 y(1)],'Color',[1 0 0]);
% move the pendulum
for k=2:size(x,1)
% pause the animation for 0.01 seconds
pause(0.01);
% change the x and y data (coordinate) for the pendulum
set(plotHandle,'XData',x(k),'YData',y(k));
% change the "string" so that it swings with the pendulum
set(lineHandle,'XData',[0 x(k)],'YData',[0 y(k)]);
end
You should be able to use the above for your use of markers.
I did notice that you defined a local variable as length. This conflicts with a built-in MATLAB function of the same name, so I suggest you rename your local variable to something else i.e. strLength.
