Example 1.  Solve the second order  I.V.P.  
    [Graphics:Images/PendulumMod_gr_21.gif]  with  [Graphics:Images/PendulumMod_gr_22.gif],   [Graphics:Images/PendulumMod_gr_23.gif].    
Use the Runge-Kutta method to compute the solution over the interval
[Graphics:Images/PendulumMod_gr_24.gif].  

Solution 1.

The D. E. can be written as   [Graphics:../Images/PendulumMod_gr_25.gif]  where  [Graphics:../Images/PendulumMod_gr_26.gif].  
We use the substitution [Graphics:../Images/PendulumMod_gr_27.gif]  and make the second order I.V.P. into a system of first order I.V.P.'s  

    [Graphics:../Images/PendulumMod_gr_28.gif],  
and  
    [Graphics:../Images/PendulumMod_gr_29.gif],  
where  
    [Graphics:../Images/PendulumMod_gr_30.gif],  
and  
     [Graphics:../Images/PendulumMod_gr_31.gif],   [Graphics:../Images/PendulumMod_gr_32.gif].  

[Graphics:../Images/PendulumMod_gr_35.gif]



[Graphics:../Images/PendulumMod_gr_36.gif]
[Graphics:../Images/PendulumMod_gr_37.gif]
[Graphics:../Images/PendulumMod_gr_38.gif]
[Graphics:../Images/PendulumMod_gr_39.gif]

Compute the Runge-Kutta solution.

[Graphics:../Images/PendulumMod_gr_40.gif]

The solution we seek is the first coordinate in the 2D system.

[Graphics:../Images/PendulumMod_gr_41.gif]

Now we can plot the solution.

[Graphics:../Images/PendulumMod_gr_42.gif]


[Graphics:../Images/PendulumMod_gr_43.gif]

[Graphics:../Images/PendulumMod_gr_44.gif]
[Graphics:../Images/PendulumMod_gr_45.gif]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(c) John H. Mathews 2004