Function hertz_contact_time 

pub fn hertz_contact_time(
    mass: f32,
    eff_young: f32,
    rel_velocity: f32,
    radius: f32,
) -> f32

Approximate collision time from material properties (Hertz contact) t_contact ~ 2.87 * (m / (E_eff * v_rel))^(2/5)