Variable might be set by a nonscalar operator

what is wrong with variable theta_deg,

clc
clear
close all
%% Define varaibles
theta_deg=10:1:80; %% theta in 1 degrees increments
theta_rad=deg2rad(theta_deg);
theta_decline= 40.65;
thata_incline=18.05;
M=1; % mass, kg
g=9.81; %acceleration 
m_M=0.5;
Mu_s=0.2;
Mu_k=0.15;
%F_f_k=Mu_k*M*g*cos(theta_deg);  % dynamic friction force
%F_f_s=Mu_s*M*g*cos(theta_deg); % static friction force
if (10 <theta_deg) && (theta_deg<=18.05)
    F_f_k=Mu_k*M*g*cos(theta_deg);  % dynamic friction force, box moving upwards
end
if (18.05 <theta_deg)&&(theta_deg<=40.65)
    F_f_s=Mu_s*M*g*cos(theta_deg); % static friction force, the box is still
end
if    (40.6<theta_deg)&&(theta_deg<=80)
    F_f_k=Mu_k*M*g*cos(theta_deg);  % dynamic friction force, the box is sliding downwards
end

Answer 1

[Torsten]

%% Define varaibles
theta_deg=10:1:80; %% theta in 1 degrees increments
theta_incline=18.05;
theta_decline= 40.65;
M=1; % mass, kg
g=9.81; %acceleration 
Mu_s=0.2;
Mu_k=0.15;
theta_zone1 = theta_deg<=theta_incline;
theta_zone2 = theta_deg >theta_incline & theta_deg<=theta_decline;
theta_zone3 = theta_deg >theta_decline ;
F_f_k(theta_zone1) = Mu_k*M*g*cosd(theta_deg(theta_zone1));
F_f_k(theta_zone2) = Mu_s*M*g*cosd(theta_deg(theta_zone2));
F_f_k(theta_zone3) = Mu_k*M*g*cosd(theta_deg(theta_zone3));
plot(theta_deg,F_f_k)
grid on

●

[Saif]

thank you, what about a grapth for theat_deg vs F_f_s?

[Torsten]

I doesn't make sense to use different arrays for different zones for the variable theta_deg. All should be saved in one array (F_f_k) as I did above. As you can see, there are jumps in the friction force curve at theta_deg = 18.05° and theta_deg = 40.65°.

[Saif]

Make sense, thank you!

Answer 2

[Image Analyst]

If theta_deg is in degrees, you should be using the degrees version of cosine, not the radian version. In other words use cosd, not cos

help cosd

 cosd - Cosine of argument in degrees
    This MATLAB function returns the cosine of the elements of X, which are
    expressed in degrees.

    Syntax
      Y = cosd(X)

    Input Arguments
      X - Angle in degrees
        scalar value | vector | matrix | multidimensional array | table |
        timetable

    Output Arguments
      Y - Cosine of angle
        scalar value | vector | matrix | multidimensional array | table |
        timetable

    Examples
      openExample('matlab/Cosineof90degreescomparedtocosineof2radiansExample')
      openExample('matlab/CosineofcomplexanglesspecifiedindegreesExample')

    See also cos, acos, acosd

    Introduced in MATLAB before R2006a
    Documentation for cosd
       doc cosd

    Other uses of cosd

       codistributed/cosd    gpuArray/cosd    sym/cosd    tabular/cosd