Func returns a vector of length 2, but the length of initial conditions vector is 9.

Question

Arda Canbas 2021-11-27

1
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https://ww2.mathworks.cn/matlabcentral/answers/1597249-func-returns-a-vector-of-length-2-but-the-length-of-initial-conditions-vector-is-9

回答： William Rose 2021-11-27

%% We will create a function that refers to our system and its variables
function dx=mbk(t,x)
m=10;              % mass
b=0;               % value of spring which creates oscillation  
k=1600; 
F0=0;
sigma=1;
Fext=F0*sin(sigma*t); % harmonic function 
dx(1)=x(2);
dx(2)=1/m*(-b*x(2)-k*x(1)+Fext); % mechanical system equation
dx=dx';

this is my function,

clear
clc
% This code is going to solve our system function
tspan=[0 10];       % time interval
n= -4:4;          %initial condition
delta_n=[0,0,0,0,1,0,0,0,0]
[T, X]=ode23('mbk',tspan,delta_n); % ode23 command solves differantial equations 
displacement=X(:,1);
v=X(:,2);
%% Results 
figure(1)
plot(T,displacement)
xlabel('Time/sec')
ylabel('Displacement')
grid on
figure(2)
plot(T,v)
xlabel('Time/sec')
ylabel('Velocity')
grid on

and this one is my solution for function, im trying to give the system a unit impulse thats why i created delta_n but now it says

"mbk returns a vector of length 2, but the length of initial conditions vector is 9."

how can i solve this problem ^^

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Answer 1

William Rose 2021-11-27

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1597249-func-returns-a-vector-of-length-2-but-the-length-of-initial-conditions-vector-is-9#answer_841764

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@Arda Canbas,

Your initial conditions should be a 2-element vector, since mbk() returns a 2-componnt vector.

So do

tspan=[0 10];       % time interval
%n= -4:4;           %initial condition
%delta_n=[0,0,0,0,1,0,0,0,0]
x0=[-4,4];          %or x0=[0,1]; or whatever you wish the initial conditions to be
[T, X]=ode23('mbk',tspan,x0); % ode23 command solves differantial equations to use