Thread: Runge Kutta Projectile Problem

  1. #1
    Registered User
    Join Date
    Oct 2011

    Runge Kutta Projectile Problem

    Hi, I'm onto a second part of a problem sheet I've been given, and I've had to modify some code(that previously worked fine) to use the Runge Kutta method for solving ODE's. I have my code, shown below, but its outputting a straight line, and I dont know why. It should be a projectile parabola. I've been looking at this for hours now and can't figure out what's wrong. Any help is appreciated.

    #include <stdio.h>#include <stdlib.h>
    #include <math.h>
    const double pi = 3.141592654, g = 9.81; //define constants for easy use
    #define F(x,y) ((Uy/Ux)-(g*x)/(pow(Ux, 2))) //defining projectile function
    int main()
        FILE *myfile;
        double U, theta, Ux, Uy, h, x, y1, y2, k1, k2, k3, k4;
        printf("Enter a launch speed between 0 and 22 in m/s\n");//0 and 22 explained in report
        scanf("%lf", &U);
        printf("Enter, in degrees, the angle of elevation you wish to shoot at\n");
        scanf("%lf", &theta);
        Ux = U*cos(theta*pi/180);// x component of velocity
        Uy = U*sin(theta*pi/180);//y component of velocity
        k1 =h*F(x,y1);
        k2 =h*F(x+(h/2), y1+(k1/2));
        k3=h*F(x+(h/2), y1+(k2/2));
        k4=h*F(x+h, y1+k3);
        myfile=fopen("myoutput.txt", "w");//open file for results to be sent
            y2 = y1 + k1/6 + k2/3 + k3/3 + k4/6;
            printf("\n\n x= %lf y= %lf", x, y2);//print results in compiler
            fprintf(myfile, "\n\n %lf %lf", x,y2);//print results in file
        while (y2>0);
        return 0;

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    > #define F(x,y) ((Uy/Ux)-(g*x)/(pow(Ux, 2))) //defining projectile function
    Why doesn't this use y?

    Also, you need to be REALLY CAREFUL with macro functions.

    > k2 =h*F(x+(h/2), y1+(k1/2));
    Expands as
    > ((Uy/Ux)-(g*x+(h/2))/(pow(Ux, 2)))

    That is, you're only doing g*x, whereas it would seem you intended g*(x+(h/2))

    Write it as a proper function to begin with.
    There's no excuse for a function-like macro here.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Registered User
    Join Date
    Oct 2011
    That function was just one given to me, its a result of the chain rule to eliminate time from the projectile equations.

    I've modified my code so now the k's are
    k1 = h*(Uy/Ux)-(g*x)/(pow(Ux, 2));	k2 = h*(Uy/Ux)-(g*(x+h/2))/(pow(Ux, 2));
    	k3 = h*(Uy/Ux)-(g*(x+h/2))/(pow(Ux, 2));
    k4 = h*(Uy/Ux)-(g*(x+h))/(pow(Ux, 2));
    But im still getting a straight line, and so my y<0 condition isn't stopping the loop...

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