How can I create a runge kutta 4th order with no x variable?

I am a total beginner at matlab. I am trying to solve the following second order differential equation:

d2y/dz2=(f/2EI)(L-z)

where f=51, E=1.1*10^8, I=0.05, L=27, step size=2.7 (which is 27/10). initial conditions: y=0, y'=0 at z=0

I am reducing the order of the equation to make it first order by using p and pdot.

p1=y, 
p1dot=y'=p2

p2dot=y''=(f/2EI)(L-z)

below is the code:

function pdot=mast(t,p)

f=50; E=1.1E+08; I=0.05; L=27;

pdot=zeros(size(p));

pdot(1)=p(2);

pdot(2)=(f/2EI)(L-z);

>>p0=[0 0];

>>[t,p]=ode45('mast',[0 27],p0);

[John D'Errico]

Please don't add answers every time you wish to make a comment. That is why there is a link that will let you add a comment. I moved your answer into a comment.

Answer 1

[Walter Roberson]

You have

pdot(2)=(f/2EI)(L-z);

Rmemeber that MATLAB does not have implicit multiplication.

pdot(2)=(f/(2*E*I))*(L-z);

But now you need to figure where you are getting "z" from.

[John D'Errico]

Moved to a comment:

"Hi Walter, Thank you for your response. I forgot to include the multipication sign between the two brackets in my question. They were included in Matlab though.

Can you explain what you mean by where am I getting the z from please?"

[Walter Roberson]

You say

p1=y, 
p1dot=y'=p2
p2dot=y''=(f/2EI)(L-z)

but what is "z" in that?

Perhaps you should be using

function pdot = mast(z, p)

Notice the change from t to z