phy_com_y;
返回： Real（实数）
This read-only variable will return the y position of the
instances center of mass. This is calculated automatically based on
the density, inertia and mass of the instance as defined by the
appropriate functions.
xx = phy_com_x;
yy = phy_com_y;
The above code sets two variables to the x and y position of the instances center of mass.