Matlalb problem with two first order ODEs

[sara_87]

Homework Statement

I have two questions:
1) If i have two first order ODE y(1) and y(2) (in terms of time), i know how to plot y(1) versus time and y(2) versus time but i don't know how to plot y(2) versus y(1)

2)I have two second order ODES X''=... and Z''=... to solve this, we make the substitutions y(1)=X, y(2)=X', y(3)=Z, and y(4)=Z'
then we have two first order ODEs: y(2)'=... and y(4)'=...
I typed this into Matlab (dydt=[(the equation for y(2)'; the equation for y(4)']) but an error came up since i didnt define y(4). how do i do this?
2. Homework Equations



3. The Attempt at a Solution
1) i tried putting:
plot(y(1),y(:,2)), and i also tried plot(y(1),y(2)) but these didnt work.


Any help would be very much appreciated.
Than you.

 

[Mindscrape]

1) plot(y(:,1),y(:,2)) ought to work, assuming you just left out the iterate through all rows part in the first column.

2) Can you post your exact system of equations function?

 

[oswaler]

I would first suggest reviewing the basics of plotting in Matlab and understanding how to plot multiple variables on the same graph. This can be achieved using the "hold on" and "plot" commands, as well as specifying the x and y variables in the plot function. Additionally, it may be helpful to review the concepts of dependent and independent variables in plotting.

For the second question, it seems that the error may be due to not defining the initial conditions for the y(4) variable. In order to solve the first order ODEs, initial conditions must be specified for all variables. I would suggest double checking the equations and initial conditions, and possibly consulting with a professor or TA for further assistance.

Overall, it is important to have a strong understanding of the mathematical concepts behind the ODEs and how they relate to each other in order to effectively solve and plot the solutions. It may also be helpful to consult with a math or physics tutor for additional guidance and practice.