# i am trying to get the value of u_E using the equation, but matlab says it has error in that line which has the equation.

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%creat symbol for axial force, length, thickness, displacemnent and width

syms f l t d w

%define governing equations for parameter

stress = f./(t*w)

strain = d./l

%measurement resolution and uncertainty

u_f = (f./100)./2./12;

u_l = 1/32/12/2;

u_t = 0.001/2/12;

u_d = (d./100)./2./12;

u_w = 0.001/12/2;

%zeroth order uncertainty

E = (f.*l)./(d.*w*t);

u_E = sqrt((diff(E,u_f).*u_f).^2 + (diff(E,u_d).*u_d).^2 + ...

(diff(E,u_l).*u_l).^2+(diff(E,u_t).*u_t).^2+(diff(E,u_w).*u_w).^2)

%define symbolic data

T = readtable('Intro_specimen 2.txt');

f = T{:,2};

l = 6.125;

t = 0.057;

d = (T{:,1})./12;

w = 0.414;

%substitute value into equation

stress = eval(stress);

strain = eval(strain);

%plot with errorbar

plot(strain,stress)

hold on

title('strain vs. stress')

xlabel('strain (ft^3)')

ylabel('stress (slug/ft^3)')

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### Answers (2)

Paul
on 10 Sep 2022

Hi kaixi

%creat symbol for axial force, length, thickness, displacemnent and width

syms f l t d w

%define governing equations for parameter

stress = f./(t*w);

strain = d./l;

%measurement resolution and uncertainty

u_f = (f./100)./2./12;

u_l = 1/32/12/2;

u_t = 0.001/2/12;

u_d = (d./100)./2./12;

u_w = 0.001/12/2;

%zeroth order uncertainty

E = (f.*l)./(d.*w*t);

u_E = sqrt((diff(E,u_f).*u_f).^2 + (diff(E,u_d).*u_d).^2 + ...

(diff(E,u_l).*u_l).^2+(diff(E,u_t).*u_t).^2+(diff(E,u_w).*u_w).^2)

In the Symbolic Math Toolbox, the function diff is used to diffentiate an expression with respect to a variable. So

diff(E,u_f)

means take the derivative of E wrt u_f, but u_f is an expression, not a variable. Similarly, u_l is a number, not a variable.

What exactly is the math that we're trying to implement?

##### 2 Comments

Torsten
on 10 Sep 2022

f is the force and d is the displacement, and they are a set of data from the txt file.

And why - if they are data from a txt file - do you define them as syms ?

Walter Roberson
on 10 Sep 2022

##### 0 Comments

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