Plot call does not do its work properly.

I've been working in a project and every time I run this code in my computer it doesn't show the three graph it is suposed to graph. I have a function define and saved:

function [dydt] = spring_mass_system(t,y)

%Definition of 2nd order ODE for the spring-mass system

m = 20;

c(1)=5;

c(2)=40;

c(3)=200;

k = 20;

for i=1:3

dydt = zeros(2,1); %define colum array for output

dydt(1) = y(2);

dydt(2) = - (c(i)/m) * y(2) - (k/m) * y(1);

end

end

and I call my function by this script where I (supposedly) graph the three results in the the same picture. I run it in another computer and it does it how I want it. Any suggestion how to make this work properly, please? You can see I've added suggestion found online but, I it doesn't do anything to me.

tspan = [0,15];

ic = [0 1];

[t,y] = ode45(@spring_mass,tspan,ic);

figure;

plot(t,y(:,2));

grid;

xlabel('Time (s)');

ylabel('Dispalcement (m)');

hold on

thank you so much for your help.

Answer 1

[Star Strider]

The plot call you posted has it only plotting the second output of ‘spring_mass_system’.

It doesn’t show three lines because ‘spring_mass_system’ only returns two. That is how you wrote your ‘spring_mass_system’ ODE function.

[Lazaro Gonzalez]

But even with two values, should it show two line at least, isn't it?

[Star Strider]

It does if you change the plot call to:

plot(t,y);

Your existing code has it only plotting the second column of ‘y’.

[Lazaro Gonzalez]

You were right. I am just plotting the second function values. I changed the value of my output vector but I am not getting any changes in my answer other than graphying y (with the last code you sent) and the previous answer.

[Star Strider]

If you want to plot position, velocity, and acceleration as functions of time, you need to write your ‘spring_mass_system’ differential equations to provide them. That will give you the (Nx3) ‘y’ matrix you want.

Since this appears to be a homework assignment, I leave that to you.