physics_particle_get_damping()
返回： Real（实数）
With this function you can find out what the current density is
for particles in the physics simulation (you can set this value
using physics_particle_set_density()).
if physics_particle_get_density() < 1
{
physics_particle_set_density(physics_particle_get_density() + 0.01);
}
The above code will check the current density value for all particles in the system and if it is less than 1 then it will add 0.01 to it.