docs: def T_i(v_vars: List[fl.Var], mass: np.ndarray, i: int):
      """Make Fluxion with the kinetic energy of body i"""
      # Check that the lengths are consistent
      assert len(v_vars) == 3 * len(mass)
  
      # Mass of the body i
      m = mass[i]
      
      # kineteic energy = 1/2 * mass * speed^2
      T = (0.5 * m) * flux_v2(v_vars, i)
      return T