Function rk4_2d 

pub fn rk4_2d<F>(
    pos: Vec2,
    vel: Vec2,
    mass: f32,
    dt: f32,
    force_fn: F,
) -> (Vec2, Vec2)
where
    F: Fn(Vec2, Vec2) -> Vec2,

Runge-Kutta 4 for a 2D position given a force function.