How to Plot a second derivative equation?
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Guan-Lin Chen
le 10 Nov 2020
Commenté : Star Strider
le 10 Nov 2020
Hi all,
The equation I want to plot is
I've tried to gether information online and come up with the code:
clc
clear
syms x(t)
Dx = diff(x);
D2x = diff(x,2);
w = 1
dt = 1/1000;
t = 0:dt:1000*dt;
equ = D2x + (Dx)^3 + x(t)^2 *Dx + x(t) == 0;
figure
fplot(equ)
How to plot the graph with the time interval t?
Edit:
To plot the 2 equation and plot them on the same graph
Can I use the code like this:
syms x_1 x_2 equ(t)
Dx_1 = diff(x_1);
D2x_1 = diff(x_1,2);
Dx_2 = diff(x_2);
D2x_2 = diff(x_2,2);
w_1 = w_2 = 1
a_1 = a_2 = 1
B_1 = B_2 = 1
r_1 = r_2 = 1
equ_1 = D2x_1 + (a_1*(Dx_1) + B_1*x_1^2 - r_1 )*Dx_1 + w_1^2*x_1;
equ_2 = D2x_2 + (a_2*(Dx_2) + B_2*x_2^2 - r_2 )*Dx_2 + w_2^2*x_2;
figure
fimplicit(equ_1, [-1 1]) % Use _fimplicit1 To Plot ‘equ(t)=0’
hold on
fimplicit(equ_2, [-1 1])
grid
Would the graph plot with this code be the solution of the equation?
2 commentaires
David Wilson
le 10 Nov 2020
Are you sure this is what you want to solve? Given that you have given us the explicit solution for x(t), why not just plot that?
However, I'm not sure that this is correct. Here is your (posulated) solution for x(t):
clear
syms t real
w=1;
x = exp(-1i*w*t) + exp(1i*w*t)
Now, does this satisfy your ODE?
Dx = diff(x,t)
D2x = diff(x,2)
res = D2x + (Dx)^3 + x^2*Dx + x
r = simplify(res)
Now that gives us which does not equal zero like it should for al t.
Réponse acceptée
Star Strider
le 10 Nov 2020
Try this:
syms x(t) equ(t)
Dx = diff(x);
D2x = diff(x,2);
w = 1
% dt = 1/1000;
% t = 0:dt:1000*dt; % Not needed Here
equ = D2x + (Dx)^3 + x(t)^2 *Dx + x(t);
equ(t) = subs(equ, x, {exp(-1i*w*t)+exp(1i*w*t)}) % Substitute Expression For
figure
fimplicit(equ, [-1 1]) % Use ‘fimplicit’ To Plot ‘equ(t)=0’
grid
The plot is absolutely not interesting, however the code runs without error!
6 commentaires
Star Strider
le 10 Nov 2020
‘... and second, ‘equ_1’ and ‘equ_2’ must be functions of only one variable,. so the others must have scalar numeric values.’
That is how to fix it!
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