Vectors Are Not Lining Up

Hello,

I am having a bit of difficulty with this problem. When I run my code I get an error saying that my vectors are not the same size. Any help is greatly appreciated. The code is provided below.

function ydd=SecDeriv(x,y)
%   Calculate the size of the array x
n = length(x);
%   Calculate the spacing of the data h
h = x(2)-x(1);
%   Second derivative at first point of the array x: 4-point forward difference formula - Error = O(h^2)
ydd(1) = (2*y(1)-5*y(2)+4*y(3)-y(4))/h/h;
%   Second derivative at last point of the array x: 4-point backward difference formula - Error = O(h^2)
ydd(n) = (y(n-3)-4*y(n-2)+5*y(n-1)-2*y(n))/h/h;
%   Second derivative at the intermediate points: three point central difference formula - Error = O(h^2)
for i=2:n-1

ydd(i) = (y(i-1)-2*y(i)+y(i+1))/h/h;

end
end

This is then used in this script file:

%   Plot the exact bending moment and the approximate bending moment
%   on the same set of axes
x = [0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360];
y = [0 -0.111 -0.216 -0.309 -0.386 -0.441 -0.473 -0.479 -0.458 -0.412 -0.345 -0.263 -0.174 -0.090 -0.026 0];

L = 360; E = 29E6; I = 720; q0 = 250;

x1 = linspace(0,L,101);
Mexact = -(q0*L^4/120)*((20/L^2)*(x./L).^3-(12/L^2)*(x./L));

M = E*I*SecDeriv(x,y);
plot(x,M,'*',x1,Mexact,'--');xlabel('x (in)');ylabel('Moment (in-lb)');
legend('Mexp','Mexact');title('Bending Moment');

I am having trouble, I believe, with the line Mexact = -(q0*L^4/120)*((20/L^2)*(x./L).^3-(12/L^2)*(x./L));

Answer 1

[Star Strider]

Guessing wildly here, but since you want to plot ‘Mexact’ as a function of ‘x1’, you might want to calculate it using ‘x1’ instead of ‘x’:

Mexact = -(q0*L^4/120)*((20/L^2)*(x1./L).^3-(12/L^2)*(x1./L));

[Claire]

Yes, you are exactly right. Thank you!

[Star Strider]

My pleasure!