diff makes M less than what i want by 1 elment and i want to have second derivaitve numircal

figure(4)

E=(k.^2)*(1.0540^2)/(2*1000*9.1);

fx=diff(E)./diff(k);

fxx=fx./diff(k);

M=((1.054^2)*(1000))./fxx;

plot(k,M);

Error using plot

Vectors must be the same length.

Error in Untitled4 (line 17)

plot(k,M);

[Adam Danz]

Y = diff(X) acts on vector x by subtracting element m+1 from element m. So, if x is

x = [3 2 7 4 1]; 

then Y will be

y = [2-3, 7-2, 4-7, 1-4]
  = [-1,   5,  -3,  -3]

There will always be one less element in Y than in X.

To compute the numerical gradient at each point in x rather than between points, use

Y = gradient(X).

Answer 1

[Star Strider]

Use the gradient function.

Specifically:

k = 0:0.1:5;
figure(4)
E=(k.^2)*(1.0540^2)/(2*1000*9.1);
fx=gradient(E)./gradient(k);
fxx=fx./gradient(k);
M=((1.054^2)*(1000))./fxx;
plot(k,M);