Why can't matlab solve if a constant value is supplied?

I've made a file to calculate the maximum effective stress for a dam. Given h is the height of the caisson.

h=0:1:10 
M = ((1025*0.5*(1/3)*h^3)

When h is increased till 11 M becomes the formula as in the M-file. The error gives an error

((??? Error using ==> mupadmex
Error in MuPAD command: Invalid index [list];

during evaluation of 'matchNonSingletonLHS'

Error in ==> sym.sym>sym.subsasgn at 1420
          C = mupadmex('mllib::subsasgn',A.s,B.s,inds{:});

Error in ==> s at 14
  sol(i,1) = solve(f,x);))

clc
close all
clear all
i=1;
for 
h=11:1:30
  syms x
  M = ((1025*0.5*(1/3)*h^3)-(1025*0.5*(1/3)*((h-10)^3)));
  V = (2*(h-2)+2*x)*2400*9.81+((h-2)*(x-2))*1800*9.81-(h*x)*1025*9.81;
  f = (V/x)+((6*M)/x^2)-500;
  sol(i,1) = solve(f,x);
  i=i+1;
end
eval(sol)

What goes wrong??

Answer 1

[Walter Roberson]

I do not have that toolbox to test with, but I suggest you do not loop:

syms x h
M = ((1025*0.5*(1/3)*h^3)-(1025*0.5*(1/3)*((h-10)^3)));
V = (2*(h-2)+2*x)*2400*9.81+((h-2)*(x-2))*1800*9.81-(h*x)*1025*9.81;
f = (V/x)+((6*M)/x^2)-500;
solsym = solve(f,x);
sol = double(subs(solsym, h, 11:1:30));

Answer 2

[A Jenkins]

Your "f" does not have a real valued solution (it has imaginary components). Perhaps you have an error in your equations for M and V?

>> f = (V/x)+((6*M)/x^2)-500

f =
((381609*x)/4 + 105948)/x + 5855114166271999/(4294967296*x^2) - 500

>> solve(f,x)

ans =
 - 0.55819540632598278755245528952159 + 3.7487595937264604137316453147829*i
 - 0.55819540632598278755245528952159 - 3.7487595937264604137316453147829*i