Sorry for this very simple question but I am still very new to the laws of motion.
I am dealing with 2-dimensional vectors in my programming environment and I'm following these slides to learn about simple integrators.
Near the end of the slides for the 4th Order Runge Kutta Integrator he is calculating acceleration like this:
a2 = acceleration(p2, v2)
However, I'm not quite sure where that function is defined in the slides. I'm sure the answer is very simple but for all prior slides the acceleration was always constant.