What's wrong with my code for this plot?

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Josie
Josie 2014-8-25
评论： Star Strider 2014-8-25
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')

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采纳的回答

Star Strider
Star Strider 2014-8-25
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.
  2 个评论
Josie
Josie 2014-8-25
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
Star Strider 2014-8-25
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')

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更多回答（1 个）

Yona
Yona 2014-8-25
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.
  1 个评论
Josie
Josie 2014-8-25
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')

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