index issue, plotting curve for engineering coursework

Hello everyone,

I'm attempting to plot a function for an engineering report on balloon inflation. Im attempting to plot stretch vs pressure, but MATLAB suggests that my "index exceeds array bounds" - im not sure why, i'm just after an output value for Y across a range of input values for P. my code as below;

clear

clc

%initial parameters

%axial case

pi = 3.14153;

re = 1e-3;

rif = 0.25e-3;

l = 2e-3;

mu = 0.4;

rhoc = 2700000*re/2*(re-rif);

rhot = rhoc/2;

%equations

P = 200000:100000:2700000;

Y = (pi*rhot*(re^2-rif^2)) / (l*(pi(P)*rif-2*mu*(pi*(P)*rif+rhoc*re+rhoc*rif)));

equation is rearranged to make Y the subject, as i don't know it but have a known range of P values to use.

plot(P,Y)

Thanks

Rich

Answer 1

[Stephan]

Hi,

no need to define pi - it is a constant in Matlab already. Your issue is:

pi(P)

which is interpreted as the P'th value of pi. I used:

pi.*P

which i guessed to be meant. This gives:

%initial parameters
%axial case
re = 1e-3;
rif = 0.25e-3;
l = 2e-3;
mu = 0.4;
rhoc = 2700000*re/2*(re-rif);
rhot = rhoc/2;
%equations
P = 200000:100000:2700000;
Y = (pi*rhot*(re^2-rif^2))./(l*(pi.*P*rif-2*mu*(pi.*P*rif+rhoc*re+rhoc*rif)));
plot(P,Y)

with result:

Best regards

Stephan

[Richard Gray]

thank you very much, it works for me!

just as an aside; if i rearranged this so P was on the LHS (the original form i had this equation in), would i still be able to plot P against Y even though Y is unknown?