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MA2051  Ordinary Differential Equations
Matlab  Solve a secondorder equation numerically
In vector notation, this system has the form , where
Now use an editor to create a Mfile named frcdsprg.m for this vector system :
% frcdsprg.m  new version % x'' + 2x' + x = sin( 0.9 t ) written as the system % x' = v, v' = x  2v + sin( 0.9 t) % y(1) is x, y(2) is v. function yprime = frcdsprg( t, y ) yprime(1) = y(2); % x' = v in terms of y's yprime(2) = y(1)  2*y(2) + sin( 0.9*t ); % v' = x ... in terms of y's
The following sequence of commands will solve this system with initial conditions x(0) = 1, v(0) = 2 for .
y0 = [1; 2]; [t, y] = wrk4( 'frcdsprg', 0, 30, y0, 0.2 );The plot that appears is the graph of the approximate solution curve x(t) for .
Type [t,y] (without a semicolon). Three columns of numbers will fly by; the first column is t, the second column is y(1) = x(t), and the third column is y(2) = v(t). In the above instructions, notice that you are solving both of the firstorder equations at the same time. The initial data is specified in y0 = [1;2], and the time step by 0.2.
The following commands will plot x(t) and v(t) again st t and annotate the plot:
plot(t,y(:,1),''); % plots x(t) with lines hold on; % holds the old plot plot(t,y(:,2),'.'); % plots v(t) with dots title(' x" + 2 x'' + x = sin(0.9 t) '); % adds a title ('' prints as ') xlabel(' time '); ylabel(' position and velocity '); % label the axes text(15,1.0,' ... velocity '); % put text at (15,1.0) text(15,1.5,'  position '); % put text at (15,1.5)
Note that y(:,1) refers to the first column of y (which is x(t)) and y(:,2) refers to the second column of y (which is ). To make a new plot, type hold off.
The following commands will plot v(t) against x(t) (instead of t), producing a phase plane plot of this nonautonomous system:
plot(y(:,1),y(:,2),'.'); % plots x and v title(' Phase Plane: x" + 2 x'' + x = sin(0.9 t) '); % adds a title xlabel(' position '); ylabel(' velocity '); % label the axes
To control the axis limits, type axis( [ 0.75 1 1 1.5] ); the graph will show ``x'' values from 0.75 to 1 and ``y'' values from  1 to 1.5. Type help axis for more information.
Alternatively, to get a phase plane plot directly for an autonomous system, type pplane and enter your equations into the text box that appears.
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