Taking second derivative and solving it for when it equals 0
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Christina Kersten
el 21 de Mzo. de 2019
Comentada: Star Strider
el 22 de Mzo. de 2019
Here is the code that I have:
clear all; close all
m = xlsread('1 MgO.xlsx','Sheet1','B1:J18');
temp = m(:,6);
MgO = m(:,3);
plot(temp, MgO,'*')
xlabel('Temperature (K)'); ylabel('Shear Modulus (GPa)'); title('MgO')
hold on
% Line of best fit
p = polyfit(temp,MgO,3);
y = polyval(p,temp);
plot(temp,y,'-')
SSE=sum((MgO-y).^2); SST=sum((MgO-mean(MgO)).^2); Rsquared=1-SSE/SST;
text(100,100,sprintf('y = %.3x^3 + %.3fx^2 + %.3fx + %.3f, R^2 = %.3f',p(1),p(2),p(3),p(4), Rsquared))
I'm trying to figure out how to get the second derivative and find at what temperature it equals 0. temp is my xaxis and MgO is my yaxis. I tried using the diff command but it didn't really work. Could someone help please?
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Star Strider
el 21 de Mzo. de 2019
I am not certain what you are doing. Taking the derivative of data will significantly amplify any noise that might be present, so in general, it is best to take the derivative of a smooth signal. Also, you are fitting a 3° polynomial, so the second derivative of that (regardless of the method you use) is giong to be a linear (1°) function.
If you want to take the derivative of filtered data (eliminating as much noise as possible first), use the gradient (link) function. Use it two times to get the second derivative.
If you want to take the derivative of your fitted function ‘p’, one option is to use the polyder (link) function, again twice. You can then easily solve for the x-intercept of the linear relation this produces.
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Star Strider
el 22 de Mzo. de 2019
It should be:
xint = -d2p(2)/d2p(1);
That was what I intended to write, and I thought that was what I wrote.
The derivation comes from:
y = d2p(1)*x + d2p(2)
0 = dp2(1)*xint + d2p(2)
with the x-intercept defined by the point where y=0.
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