Trouble plotting multiple graphs

I trying to plot multiple graphs of mass vs drag. I know the equation is correct but the plot's shouldn't be coming out with negative drag. Is there something wrong with the code?

M=linspace(0,1,100);
g=10;
for A=0.1:0.1:0.9
    s=(A-0.5)*M.*g-(A^2)/4;
    theta=s./(2*M.*(A-0.5)^2+2*M+(A^2-A+1/3)/2);
    D=(-A.^2+A-1/3).*theta-0.5*A.^2;
    plot(M,D)
    axis([0,1,-2,2])
    hold all
end
legend('A=0.1','A=0.2','A=0.3','A=0.4','A=0.5','A=0.6','A=0.7','A=0.8','A=0.9')

[Patrik Ek]

The equation for D is quite simple assuming theta is an independent variable, which it of course not is, since it is calculated. Still it can be treated as such to calculate D(theta), since D does not rely on M except for the dependence in theta. You can look at equation D as a function of theta and find out how D varies with theta.

This can be done by creating a vector theta = -2*pi:0.01:2*pi and use it as input to D. By plotting

plot(theta,D);

you will then see how D depends on theta for different values of A. With the help of these graphs you can see if the result is reasonable.

Answer 1

[Adam]

Well, the plots are negative because the result data is.

(-A.^2+A-1/3).*theta

ranges from -0.3785 to 0.4050.

0.5*A.^2

is 0.4050. So when you subtract this from the above the highest value you get will be 0, everything else will be negative

Answer 2

[Iain]

Theres something wrong with your sums.

By hand, calculate D, for A = 0.1 and M = 0 & 100. You ought to see that D is negative.