Hey guys, I'm working on this n-body planetary system simulation and I was just wondering if anyone was familiar enough with the Runge-Kutta(RK4) approximation method that they understood how to apply it to approximating the velocity and position of a planet as it orbits around some central star. The problem I'm having with incorporating the method into my simulation is that the acceleration (the derivative of the variable I'm trying to approximate) is not a function of time. Instead, it is being treated as constant (acceleration = total_force / mass). Since, I'm not really all that knowledgeable in physics, especially astrophysics, I have absolutely no idea how to define this acceleration as a function of time or even if I really need to in order to use the Runge-Kutta method in my simulation. Any help would be greatly appreciated. Thanks.