MATLAB: Dsolve problem

dsolve

Hello, I would like to use a function f in a differential equation for example f=x but get a crazy result.

syms x y f
f=x
y=dsolve('D2x=-f','x(0)=1','Dx(0)=0')

How can I put into dsolve a function f defined outside of dsolve?

Best Answer

One way would be to convert f to a string:

syms x y f
f=x
y=dsolve(['D2x=-' char(f)],'x(0)=1','Dx(0)=0')

Related Solutions

MATLAB: I can’t plot 2nd order differential equation

I can!

Try this:

syms x(t);
Dx = diff(x);
Dx2 = diff(x,2);
x(t) = dsolve(Dx2+(5*Dx)+(6*x)==100,x(0)==0,Dx(0)==0)
figure(1)
fplot(x, [0 10])
axis([xlim    0  17])
grid

MATLAB: Using dsolve to solve a system of differential equations analytically

The differential equation system is nonlinear, so it likely does not have an analytic solution.

Try this instead:

m                     = 3.125*0.001;                   % Mass of the body in slug
Cd                   = 4.71*10^-7;                   
g = 32.2;    
syms x(t) y(t) T Y                                      
Dx = diff(x,1);                                    
D2x = diff(x,2);                                    
Dy = diff(y,1);                                  
D2y = diff(y,2);                                  
[VF,Sbs] = odeToVectorField([-Cd*Dx*sqrt(Dx^2+Dy^2)==m*D2x,-Cd*Dy*sqrt(Dx^2+Dy^2)-m*g==m*D2y],...
                  [x(0)==0  ,Dx(0)== 186*8.8/6*cosd(11.2), y(0) == 0, Dy(0)==186*8.8/6*sind(11.2)]);  
Fcn = matlabFunction(VF, 'Vars',{T,Y});
[T,Y] = ode45(Fcn, [0 10], [0 0 0 1]); 
figure
plot(T, Y)
grid
legend(string(Sbs), 'Location','SW')

Best Answer

Related Solutions

MATLAB: I can’t plot 2nd order differential equation

MATLAB: Using dsolve to solve a system of differential equations analytically

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