protected Grid grid;
//map of each *moving* body to cells it currently intersects
protected Map<Body, Set<GridCell>> currentCells;
public Phys2DGridWorld(int iterations, float timestep, int width, int height, double cellWidth, double cellHeight ) {
super(iterations, new BruteCollisionStrategy(), timestep);
grid = new Grid(width, height, cellWidth, cellHeight);
currentCells = new HashMap<Body, Set<GridCell>>(15);
}