phy_inertia;
返回： Real（实数）
Inertia is the measure of how hard it is to make something start
or stop moving, so the lower the value for this read-only
variable and the easier it will be to set the instance in motion,
while higher values will require more force to start it moving.
physics_mass_properties(70, -10, -10, phy_inertia);
The above code will change the mass and center of mass of the instance while maintaining the inertia value.