What's wrong with my code for this plot?

I'm trying to plot drag D against mass M for various values of the ratio R between the inertia I and mass M, i.e. R=I/M but when I plot this I only get the R=0.5 line. How do I fix this?

M=linspace(0,1,50);
I=linspace(0,1,50);
A=0.7;
g=10;
for R=[0.02, 0.1, 0.2, 0.5];
    R=I/M;
    s=(A-0.5)*M.*g-(A^2)/4;
    theta=s./(2*M.*(A-0.5)^2+2*I+(A^2-A+1/3)/2);
    D=(A.^2-A+1/3).*theta+0.5*A.^2;
    plot(M,D)
    axis([0,1,-1,1])
    xlabel('M')
    ylabel('D')
    hold all
    grid on
end
legend('R=0.02','R=0.1','R=0.2','R=0.5')

Answer 1

[Star Strider]

One possible problem is that R isn’t used to calculate D (or anything else), and never changes because:

R=I/M;

in every iteration of the loop. D doesn’t change either.

[Josie]

Thanks, could I instead overlap 4 different graphs on top of each other like this but then including I=0.1*M, I=0.2*M, I=0.5*M as the overlapping graphs? How would I do this?

M=linspace(0,1,50);
A=0.7;
g=10;
I=0.02*M;
    s=(A-0.5)*M.*g-(A^2)/4;
    theta=s./(2*M.*(A-0.5)^2+2*I+(A^2-A+1/3)/2);
    D=(A.^2-A+1/3).*theta+0.5*A.^2;
    plot(M,D)
    axis([0,1,-1,1])
    xlabel('M')
    ylabel('D')
    hold all
    grid on
legend('R=0.02','R=0.1','R=0.2','R=0.5')

[Star Strider]

I am still not certain what you are doing, but this gives you three lines, one for each value of Im:

M=linspace(0,1,50);
A=0.7;
g=10;
Im = [0.1  0.2  0.5];
for k1 = 1:length(Im)
    I = Im(k1)*M;
    s=(A-0.5)*M.*g-(A^2)/4;
    theta=s./(2*M.*(A-0.5)^2+2*I+(A^2-A+1/3)/2);
    D=(A.^2-A+1/3).*theta+0.5*A.^2;
    plot(M,D)
    axis([0,1,-1,1])
    xlabel('M')
    ylabel('D')
    hold all
    grid on
end
legend('R=0.02','R=0.1','R=0.2','R=0.5')

Answer 2

[Yona]

you change R in the loop to be same each time (R=I/M;) so you get the exactly same graph 4 time.

they are each on other.

by the way, axis, xlabel, ylabel and grin can be after the loop, it not need to be in the loop.

[Josie]

Thanks, could I instead overlap 4 different graphs on top of each other like this but then including I=0.1*M, I=0.2*M, I=0.5*M as the overlapping graphs? How would I do this?

M=linspace(0,1,50);
A=0.7;
g=10;
I=0.02*M;
    s=(A-0.5)*M.*g-(A^2)/4;
    theta=s./(2*M.*(A-0.5)^2+2*I+(A^2-A+1/3)/2);
    D=(A.^2-A+1/3).*theta+0.5*A.^2;
    plot(M,D)
    axis([0,1,-1,1])
    xlabel('M')
    ylabel('D')
    hold all
    grid on
legend('R=0.02','R=0.1','R=0.2','R=0.5')