Thread: Runge Kutta IV project problem

I don't have time to try your code now, but this problem should be the same as a pendulum (massless rod of length R with point mass at end of rod). You can do a web search to find exact equations for pendulums and compare the results to your programs output.

If I compile your program from post #5 I get the following warnings:

Code:

$ gcc -Wall -Wextra -ggdb3 foo.c -lm
foo.c: In function ‘fun’:
foo.c:73:19: warning: unused parameter ‘x0’ [-Wunused-parameter]
foo.c: In function ‘main’:
foo.c:107:5: warning: passing argument 5 of ‘vrk4’ from incompatible pointer type [enabled by default]
foo.c:20:10: note: expected ‘void (*)(double,  double *, double *)’ but argument is of type ‘double (*)(double,  double *, double *)’
foo.c:112:5: warning: passing argument 5 of ‘vrk4’ from incompatible pointer type [enabled by default]
foo.c:20:10: note: expected ‘void (*)(double,  double *, double *)’ but argument is of type ‘double (*)(double,  double *, double *)’
foo.c:99:11: warning: unused variable ‘j’ [-Wunused-variable]
foo.c:99:9: warning: unused variable ‘p’ [-Wunused-variable]
foo.c:98:12: warning: unused variable ‘ss’ [-Wunused-variable]
foo.c:94:12: warning: unused variable ‘F’ [-Wunused-variable]
foo.c:135:1: warning: control reaches end of non-void function [-Wreturn-type]
foo.c: In function ‘fun’:
foo.c:81:1: warning: control reaches end of non-void function [-Wreturn-type]

You need to fix them before you can check whether your program produces a correct result or not.

Bye, Andreas

Based on the math from pendulums, since your function for angular acceleration is g sin(theta) / r, then if the initial condition is theta = pi/2 and omega = 0, then the time it should take to cycle back to pi/2 will be about 1.18 * 2 * pi * sqrt(r / g). In your case, with r = 0.5 and g = 9.81, the cycle time should be about 1.674 seconds. You can use this information to confirm your results.

I still can't get through the -1.#IND00 ... i'm not too familiar with programming at all, and I can't find the problem computer does... I changed the quantities of thetha and omega but still can't get the program working :/

okay, i have another question. I got through the problem, the program is working correctly, but i wonder if the mechanic energy should be constant?

wow, can you help me out with total energy formula? because mine ( m*(fabs(y1[1])*R)*(fabs(y1[1])*R)/2.)+m*g*R*cos(y1[0]) doesn't work as a constant...

and also, how to calculate angular path? not angular position which right now is calculated.

Try setting your time step smaller, like "t = .0335;" . This goes through 2 cycles in 100 steps: (~ 2 * 1.674 seconds / 100).