Im trying to write code for a vertically falling cannonball, i keep getting an error could "Undefined function 'function name' for input arguments of type 'double'. Can anyone help me out?

 function [v,vterm] = Student(m,d,T)
 % A funuction to deterined a vertically falling cannonball, while being
 % effected by drag

 % Various Constant
 cd = 0.47; % drag coefficient for a sphere
 rho = 1.2; % density of air
 g = 9.81;  % gravitational constant

 dt = 0.1; % Time step
 v = 0;    % Vertically falling cannonball, released from rest
 for t = 0:dt:T
    a = ((cd*rho*(pi*d^2))/2*m)-g ; 
    v = v(t-1) + a(t)*dt ;
    hold on; 
    plot (t,v,'.');
 end

 Fnet = cd*0.5*rho*(v(t)^2)*(pi*d^2);
 if Fnet == 0
    vterm = sqrt(2*m*g/rho*(pi*d^2)*cd);
 end

The code I used to call this function is as followed

 [v,vterm] = Student(5,2,20)

and the error message that was returned to me was:

Undefined function 'Student' for input arguments of type 'double'.

%

[Stephen23]

Please edit youir question, and give us the following information:

• the exact code that you use to call this function.
• the exact and complete error message. This mean all of the red text.

[Dominic Gladdish]

I have added a few things to my question hopefully you can understand what Im asking for a bit better now

Thank you for your time and help

[Image Analyst]

That's not error I get. I can call the function problem - it's inside it where there's an error. I get that it doesn't know about v(-1) which is exactly what Walter told you about. Did you not see his answer about how to correct the code?

[Dominic Gladdish]

I have seen what Walter has said, and i dont feel my code is right which is why i put the photo up as well, i have taken out the v(t-1) etc so that line of the code just looks like

 v = v + a*dt ;

but I still recieve the same error message as before.

Is there some code I need to take out? Is there any code I need to add to help matlab return both (v,vterm) outputs?

Answer 1

[Walter Roberson]

Your line

      a = ((cd*rho*(pi*d^2))/2*m)-g ;

computes a scalar value, unless d happens to be a square 2D array or m happens to be a non-scalar numeric value.

Your line after that,

v = v(t-1) + a(t)*dt ;

attempts to access that scalar a at index t where t is from the for loop, a non-integer value from 0 in increments of 0.1. It is not legal to index by a floating point value. You also attempt to access the scalar v at index t-1 which would be at negative indices until t reached 1.

Referring back to your line

a = ((cd*rho*(pi*d^2))/2*m)-g

I wonder whether you are intending to divide by 2 and multiply the result by m (as your current code does), or if you are instead intending to divide by (2 times m)? The code might be valid as it is, but it is better to avoid potential confusion by rewriting the code. For example you do not need any () on that line unless you are intending to divide by (2 times m).

[Walter Roberson]

If you are getting "Undefined function 'Student' for input arguments of type 'double'." then you have not saved the code into Student.m in a directory that is on your MATLAB path. (Your current directory is automatically on your MATLAB path, but you might not have saved the file there.)