Why am I getting this error?

L = 2;
W = 2;


Ra = (W*L*(1/2));
Ma = (W*(L+L*(2/3)));

x_true = linspace (0,2*L,51);
v = ones(1,length(x_true));
m = ones(1,length(x_true));

for a = 1: (length(x_true))
x = x_true(a);
if x<=L
v(a) = Ra;
m(a) = Ra*x - Ma;

elseif x<=(3/2)*L
v(a) = Ra - (((x-L)*(x-L))/2);
m(a) = Ra*(x-L) - (((x-2)^3)/6) - Ma;

else
v(a) = Ra - (x-L)*(2*L-x) + (((2*L-x)*(W-(x-L)))/2);
m(a) = -Ma + Ra(2*L-x) - ((x-L)*(2*L-x) + (((2*L-x)*(W-(x-L)))/2))*(((2*L-x)*(W + 2*x - L))/(3*(W + x - L)));
end
end

Array indices must be positive integers or logical values.

plot (x_true,v,'r');
title('Shear Force Diagram');
xlabel('Beam Length(m)');
ylabel('Shear (N)');
xlim ([0 2.5*L])
xticks ([0 1*L 1.5*L 2.5*L])
grid on

plot (x_true,m,'b');
title('Bending Moment Diagram');
xlabel('Beam Length(m)');
ylabel('Moment (N-m)');
xlim ([0 2.5*L])
xticks ([0 1*L 1.5*L 2.5*L])
grid on

Beam_Length=x_true.';
Shear=v.';
Bending_Moment=m.';

Shear_Moment_table=table(Beam_Length,Shear,Bending_Moment);
display(Ma)
display(Ra)

Answer 1

[Dyuman Joshi]

There is a missing operator between Ra and (2*L-x) in the else statement block -

Update the code accordingly.

%%           vvvvvvvv  
m(a) = -Ma + Ra(2*L-x) - ((x-L)*(2*L-x) + (((2*L-x)*(W-(x-L)))/2))*(((2*L-x)*(W + 2*x - L))/(3*(W + x - L)));

Also, it would be better to defined the number of points used to define the variable x_true by linspace() and use that variable instead of calling length() everytime.

[AStudent]

Thank you so much, I totally missed the *. Also, I have to keep the length in terms of L and x because of the type of problem. This is supposed to represent a V and M diagram of a beam using the method of sections.

I changed my code to this, and the moment graph that it's giving me is almost right, its just the middle section (between 2<x< 3) that is wrong. I attached a picture of what the graph is supposed to look like. Any idea why it's not working?

L = 2;
W = 2;


Ra = (W*L*(1/2));
Ma = (W*(L+L*(2/3)));

x_true = linspace (0,2*L,51);
v = ones(1,length(x_true));
m = ones(1,length(x_true));

for a = 1: (length(x_true))
    x = x_true(a);
    if x<=L
        v(a) = Ra;
        m(a) = Ra*x - Ma;

    elseif x<=(3/2)*L
            v(a) = Ra - (((x-L)*(x-L))/2);
            m(a) = Ra*(x-L) + (((x-2)^3)/6) ;

    else
        v(a) = Ra - (x-L)*(2*L-x) + (((2*L-x)*(W-(x-L)))/2);
        m(a) =  - Ra*(2*L-x) + ((x-L)*(2*L-x) + (((2*L-x)*(W-(x-L)))/2))*(((2*L-x)*(W + 2*x - L))/(3*(W + x - L)));
    end
end



plot (x_true,v,'r');
title('Shear Force Diagram');
xlabel('Beam Length(m)');
ylabel('Shear (N)');
xlim ([0 2.5*L])
xticks ([0 1*L 1.5*L 2.5*L])
grid on

plot (x_true,m,'b');
title('Bending Moment Diagram');
xlabel('Beam Length(m)');
ylabel('Moment (N-m)');
xlim ([0 2.5*L])
xticks ([0 1*L 1.5*L 2.5*L])
grid on

Beam_Length=x_true.';
Shear=v.';
Bending_Moment=m.';

Shear_Moment_table=table(Beam_Length,Shear,Bending_Moment);
display(Ma)
display(Ra)

[Dyuman Joshi]

Could you please share the description and any related information of the problem you are trying to solve?

[AStudent]

For L I chose 4m, for Wo I made it 2kN-m.

Answer 2

[Image Analyst]

See the FAQ for a thorough discussion:

Subscript indices must either be real positive integers or logicals