Force models applied to collision regime II. The quasi-static drag force acting on the projectile f(z) varies linearly when 0 < z < z₁ but is constant when z₁ < z < z_stop,II. m is the mass of the projectile, g is gravity, z is the depth of the projectile, z₁ is the depth where f(z) becomes constant, z_stop,II is the final penetration depth in collision regime II, and f_o, and h are constants.