ISAAC is a star system simulator, with a force-based physics engine at its heart.
ISAAC's core layer approximates the way objects move, based on the forces acting upon them.
It's based entirely around two formulae.
We calculate an object's acceleration based on its mass and the force acting upon it, calculate its velocity based on its acceleration, then calculate a new position for the object based on its velocity.
ISAAC can simulate star systems given a simple set of specifications. So we built a model of the Solar System with it.
Change the Gravitational Constant. Fiddle with the mass of things. See how things react.