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
phy_inertia;
Real (single precision floating point value, or undefined if the instance is not physics enabled)
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.