Kindly find error in the code

Hey all!!

I have to 2 doubly differential dependant equations using ode45 function. My code is giving me values for 1st depensant variable but not giving any value for 2nd one. Kindly help me fix my code

here is my code:

tRange = [0,10]
g = 9.8
l = 1
k = 0.00006
%x1 = diff(x,t)

[tsol, ans1] = ode45(@pend,tRange,[0.05;0;0;0])

function dYdt = pend(t,Y)
  % extracting variables from Y column vaector
  x = Y(1)
  x1 = Y(2)
  y = Y(3)
  y1 = Y(4)
  % some constants defined
  g = 9.8
  l = 1
  k = 0.000051
  % Four differential eqns because 2 doubly differential equations.
 dxdt = x1
 dx1dt = -(g/l)*x + k*y1
 dydt = y1
 dy1dt = -(g/l)*y - k*x1 
 % arranging all in column vector
 dYdt = [dxdt;dx1dt;dydt;dy1dt]
end

Answer 1

[Star Strider]

Your code runs for me without error. Note that the magnitudes of columns 3 & 4 are about 1/1000 the amplitude fo columns 1 & 2, so they plot essentially as straight lines near 0. Multiplying them by 1000 will allow you to see their structure:

figure
plot(tsol, ans1.*[1 1 1000 1000])
grid

.

[Mrudul Agrawal]

is there any autoscale option in plot fxn?

[Star Strider]

There is no autoscale. The best option in that situation would be yyaxis.

Example —

figure
yyaxis left
plot(tsol, ans1(:,[1 2]))
yyaxis right
plot(tsol, ans1(:,[3 4]))
grid
legend('x','x_1','y','y_1')

You can experiment with different line styles as well.

Use the 'Location' name-value pair to put the legend where you want it.