Energy
val kinetic : Body.t -> float
val momentum : Body.t -> Vec2.t
val angular_momentum : around:Vec2.t -> Body.t -> float
val gravity : g:float -> Body.t -> float
val gravitation : gm:float -> center:Vec2.t -> Body.t -> float
val spring : k_over_m:float -> anchor:Vec2.t -> Body.t -> float