Sorry for this very simple question but I am still very new to the laws of [motion](http://en.wikipedia.org/wiki/Kinematics).

I am dealing with very simple 2-dimensional vectors in my programming environment and I'm following [these slides][1] 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.

  [1]: http://www.richardlord.net/presentations/physics-for-flash-games