# Solving two second order ODEs

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Jake Barlow on 25 Dec 2021
Commented: Star Strider on 25 Dec 2021
Hi there!
I am trying to solve for U(t) and V(t) for the two second order ODEs and ,
where a and w are constants and with the initial conditions and .
Then I want to plot the solutions against for a given time interval.
syms U(t) V(t)
%Constants definition
a = 1;
w = 100;
dU=diff(U,t);
dV=diff(V,t);
%Initial Conditions
y0 = [0 0 1 1];
eq1 = diff(U, t, 2) == -a*w*dV;
eq2 = diff(V,t, 2) == a*w*dU;
vars = [U(t); V(t)]
OTV = odeToVectorField([eq1,eq2])
M = matlabFunction(OTV,'vars', {'t','Y'});
interval = [0 5]; %time interval
ySol = ode45(M,interval,y0);
tValues = linspace(interval(1),interval(2),1000);
yValues1 = deval(ySol,tValues,1); %U(t) solution
yValues2 = deval(ySol,tValues,2); %V(t) solution
plot(yValues1,yValues2)
Does the above code correctly solve the system of differential equations and initial conditions and plot V(t) against U(t)?
If not, then please let me know what is incorrect. Also plese let me know if there is another way to solve the system of ODEs.
Thank you for your help. Much appreciated.

Star Strider on 25 Dec 2021
Essentially, yes.
However it could be made a bit more efficient —
syms U(t) V(t)
%Constants definition
a = 1;
w = 100;
dU=diff(U,t);
dV=diff(V,t);
%Initial Conditions
y0 = [0 0 1 1];
eq1 = diff(U, t, 2) == -a*w*dV;
eq2 = diff(V,t, 2) == a*w*dU;
vars = [U(t); V(t)]
vars = [OTV,Subs] = odeToVectorField([eq1,eq2])
OTV = Subs = M = matlabFunction(OTV,'vars', {'t','Y'});
interval = [0 5]; %time interval
ySol = ode45(M,interval,y0);
tValues = linspace(interval(1),interval(2),1000);
yValues1 = deval(ySol,tValues,1); %U(t) solution
yValues2 = deval(ySol,tValues,2); %V(t) solution
plot(yValues1,yValues2) % ---------- Slightly More Efficient: Solves Directly & Avoids The 'deval' Calls ----------
interval = [0 5]; %time interval
tValues = linspace(interval(1),interval(2),1000);
[t,y] = ode45(M,tValues,y0);
yValues1 = y(:,1); %U(t) solution
yValues2 = y(:,2); %V(t) solution
figure
plot(yValues1,yValues2)
xlabel('$V(t)$', 'Interpreter','latex')
ylabel('$\frac{dV(t)}{dt}$', 'Interpreter','latex') .
Star Strider on 25 Dec 2021
My pleasure!

Paul on 25 Dec 2021
I think there are a few mistakes in the code
syms U(t) V(t)
%Constants definition
a = 1;
w = 100;
dU=diff(U,t);
dV=diff(V,t);
%Initial Conditions
y0 = [0 0 1 1];
eq1 = diff(U, t, 2) == -a*w*dV;
eq2 = diff(V,t, 2) == a*w*dU;
vars = [U(t); V(t)]
vars = [OTV,S] = odeToVectorField([eq1,eq2]);
S
S = Note that S is ordered [V dV U dU], so that should be the ordering of the solution of ode45
M = matlabFunction(OTV,'vars', {'t','Y'});
interval = [0 5]; %time interval
% note the IC's are in the same order as S
ySol = ode45(M,interval,[0 1 0 1],odeset('MaxStep',0.0001,'InitialStep',0.0001));
tValues = linspace(interval(1),interval(2),10000);
yValues1 = deval(ySol,tValues,1); % V(t) solution
yValues2 = deval(ySol,tValues,2); % Vdot(t) solution
yValues3 = deval(ySol,tValues,3); % U(t) solution
yValues4 = deval(ySol,tValues,4); % Udot(t) solution
figure
plot(yValues3,yValues1) % plot U vs V
xlabel('U');ylabel('V') An exact solution can be computed using dsolve()
sol = dsolve([eq1; eq2],[U(0)==0; V(0)==0; dU(0)==1; dV(0)==1])
sol = struct with fields:
V: sin(100*t)/100 - cos(100*t)/100 + 1/100 U: cos(100*t)/100 + sin(100*t)/100 - 1/100
Ufunc = matlabFunction(sol.U);
Vfunc = matlabFunction(sol.V);
plot(Ufunc(tValues),Vfunc(tValues))
xlabel('U');ylabel('V') % compare numerical and exact solutions
figure
plot(tValues,yValues3-Ufunc(tValues),tValues,yValues1-Vfunc(tValues)) Jake Barlow on 25 Dec 2021

R2021b

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