How to change only certain numbers in a matrices for a certain iteration?

Hello, im struggling to understand a problem. I have solved a truss analysis by using the method of joints and turned it into a 10x10 matrix. I am solving using Ax=B form. I need to be able to find when the truss fails for a certain ultimate force at a certain m. therefore in the matrix B = [0;0;0;0;0;w;0;w;0;0]; w needs to increase from 2kg to m_max incrementing by .5 kg. This is what i have so far. I am extremely new to matlab.

A = [1	0	0	0	0	0	1	0	0	0;
    0	0	0	0	0	0	0	1	0	0;
    -1	0.707	0	0	-0.707	0	0	0	0	0;
    0	-0.707	0	0	-0.707	-1	0	0	0	0;
    0	-0.707	-1	0	0	0	0	0	0	0;
    0	0.707	0	0	0	0	0	0	0	0;
    0	0	1	-1	0	0	0	0	0	0;
    0	0	0	0	0	1	0	0	0	0;
    0	0	0	1	0.707	0	0	0	1	0;
    0	0	0	0	0.707	0	0	0	0	1];

F_ult = 400
B = [0;0;0;0;0;w;0;w;0;0];
N = 100;
for w = 2:.5:N
    F = A\B;
    while F<<400
    end 

Answer 1

[KSSV]

A = [1	0	0	0	0	0	1	0	0	0;
    0	0	0	0	0	0	0	1	0	0;
    -1	0.707	0	0	-0.707	0	0	0	0	0;
    0	-0.707	0	0	-0.707	-1	0	0	0	0;
    0	-0.707	-1	0	0	0	0	0	0	0;
    0	0.707	0	0	0	0	0	0	0	0;
    0	0	1	-1	0	0	0	0	0	0;
    0	0	0	0	0	1	0	0	0	0;
    0	0	0	1	0.707	0	0	0	1	0;
    0	0	0	0	0.707	0	0	0	0	1];
F_ult = 400 ; 

N = 100;
iter = 0 ;
x = zeros(10,[]) ; % save deformations for each iteration 
for w = 2:.5:N
    iter = iter+1 ; 
    B = [0;0;0;0;0;w;0;w;0;0];  
    x(:,iter) = A\B; % save deformations for each w 
end 

Answer 2

[Walter Roberson]

A = [1	0	0	0	0	0	1	0	0	0;
    0	0	0	0	0	0	0	1	0	0;
    -1	0.707	0	0	-0.707	0	0	0	0	0;
    0	-0.707	0	0	-0.707	-1	0	0	0	0;
    0	-0.707	-1	0	0	0	0	0	0	0;
    0	0.707	0	0	0	0	0	0	0	0;
    0	0	1	-1	0	0	0	0	0	0;
    0	0	0	0	0	1	0	0	0	0;
    0	0	0	1	0.707	0	0	0	1	0;
    0	0	0	0	0.707	0	0	0	0	1];

F_ult = 400

F_ult = 400

syms w positive
B = [0;0;0;0;0;w;0;w;0;0];
F = A\B

F = 

maxF_ratio = max(abs(F/w))

maxF_ratio = 

3

breaking_w_exact = double(F_ult / maxF_ratio)

breaking_w_exact = 133.3333

breaking_w = ceil(breaking_w_exact / 0.5) * 0.5

breaking_w = 133.5000

That is, 133.0 would not be enough to break it, and you are incrementing by 0.5, so the number you are looking for is 133.5

I assumed here that a -400 force would also break the system.